L  «BRAnY  OF 


THE  WONDERS  OE  OETICS. 


THE 

WONDERS  OF  OPTICS. 


BY 

F.  MARION. 

TRANSLATED  FROM  THE  FRENCH,  AND  EDITED  BY 

CHARLES  W.  QUIN,  F.C.S. 


ILLUSTRATED  WITH  SEVENTY  ENGRAVINGS  ON  WOOD, 
AND  A  COLOURED  FRONTISPIECE. 


NEW  YORK: 
CHARLES  SCRIBNER'S  SONS, 

SUCCESSORS  TO 

SCRIBNER,  ARMSTRONG,  &  CO. 


PREFACE. 


The  present  work  needs  but  little  introduction  to 
the  English  public.  The  author,  M.  F.  Marion,  who 
holds  a  high  official  scientific  position  in  Paris,  is  well 
known,  especially  in  Europe,  as  a  popular  writer  on 
the  "Wonders  of  Optics,"  and  kindred  subjects.  As  a 
rule,  the  original  text  has  been  strictly  adhered  to  by 
the  Translator,  but  in  a  few  instances  certain  anecdotes 
of  a  local  character  have  been  altered  so  as  to  be  more 
generally  applicable,  or  condensed  to  make  room  for 
the  chapter  on  the  Spectroscope,  which  is  entirely 
original. 


CONTENTS. 


PAKT  1 

THE  PHENOMENA  OF  VISION. 
CHAPTER  I. 

PA«B 


THE  EYE  »    •    •  •    ••»•••  15 

CHAPTER  II. 

THE  STRUCTURE  OF  THE  EYE    .     .  22 

CHAPTER  III. 

THE  ERRORS  OP  THE  EYE  30 

CHAPTER  IV. 

OPTICAL  ILLUSIONS  •    .     •     .  36 

CHAPTER  V. 

THE  APPRECIATION  OF  COLOUR  44 


ix 


X  CONTENTS. 

CHAPTER  VI. 

ILLUSION'S  CAUSED  BY  LIGHT  ITSELF  

CHAPTER  VII. 

THE  INFLUENCE  OF  THE  IMAGINATION 


PART  II. 
THE  LAWS  OF  LIGHT. 
CHAPTER  L 

WHAT  IS  LIGHT?  •••••••••• 

CHAPTER  II. 

THE  SOLAR  SPECTRUM  •     o     •  . 

CHAPTER  III. 

OTHER  CAUSES  OF  COLOUR   . 

CHAPTER  IV. 

LUMINOUS,  CALORIFIC,  CHEMICAL,  AND  MAGNETIC  PROPERTIES 
OF  THE  SPECTRUM  • 

CHAPTER  V. 


THE  LAWS  OF  REFLECTION.- — MIRRORS 


CONTENTS. 
CHAPTER  VI. 

METALLIC  BURNING  MIRRORS  •  • 

CHAPTER  VII. 

LENSES      •  •  •••••• 

CHAPTER  VIII. 

OPTICAL  INSTRUMENTS.  THE  SIMPLE  AND  COMPOUND  MICRO- 
SCOPE.    THE  SOLAR  AND  PHOTO-ELECTRIC  MICROSCOPE  . 

CHAPTER  IX. 

THE  TELESCOPES  OF  GALILEO,  GREGORY,  NEWTON,  HERSCHEL, 
LORD  ROSSE,  AND  FOUCAULT  


PART  III. 
NATURAL  MAGIC. 
CHAPTER  L 

THE  MAGIC  LANTERN  

CHAPTER  II. 


THE  PHANTASMAGORIA 


xii 


CONTENTS. 


CHAPTER  III. 

OTHER  OPTICAL  ILLUSIONS  196 

CHAPTER  IV. 

THE  PROPERTIES  OP  MIRRORS   ..     o     .......     .  216 

CHAPTER  V 

CHINESE  SHADOWS  223 

CHAPTER  VI. 

POLYORAMA — DISSOLVING  VIEWS — DIORAMA     ......  231 

CHAPTER  VII. 

THE  STEREOSCOPE  236 

CHAPTER  VIII. 

THE  CAMERA  OBSCURA  AND  CAMERA  LUCIDA    ......  242 

CHAPTER  IX. 

THE  SPECTROSCOPE  •    .     .  249 

CHAPTER  X. 

SPECTRES — THE  GHOST  ILLUSION    .     .    •   264 


LIST  OF  ILLUSTRATIONS. 


PIG.  PAQB 

1.  Section  of  the  Eye   24 

2.  A  Camera  Obscura    27 

3.  The  Phenakistiscope   54 

4.  Disc  of  the  Phenakistiscope   55 

5.  Solar  Spectrum  Frontispiece 

6.  Absorption  of  Light  by  Sodium  Vapour    .    .  ib. 

7.  Action  of  a  Prism  on  the  Solar  Kays     .    .    .  ib. 

8.  The  Recomposition  of  Light   86 

9.  Eecomposition  of  Light  by  means  of  a  Concave  Mirror  87 

10.  Recomposition  of  Light  by  means  of  a  number  of 

Mirrors   88 

11.  Newton's  Disc   89 

12.  Newton's  Rings   95 

13  Reflection  from  Plane  Surfaces   107 

14.  Refraction  ;   108 

15.  Experimental  Proof  of  Refraction   ib. 

16.  The  Effects  of  Plane  Mirrors   109 

17.  Reflection  from  the  Surface  of  Water   110 

18.  Concave  Mirror   Ill 

19.  Conjugate  Foci   113 

20.  Virtual  Focus   114 

21.  Concave  Mirror   ib. 

22.  Magnifying  Effect  of  Concave  Mirrors   115 

23.  The  Reversal  of  real  Images   ib. 

24.  Diminishing  Power  of  Convex  Mirrors   116 

25.  Burning  Mirror   124 

26.  Double  Convex  Lens    127 

27.  Forms  of  Lenses   128 

28.  Path  of  a  Ray  through  a  Convex  Lens   129 

29.  Path  of  Divergent  Rays  through  a  Convex  Lens     .    .  ib. 

30.  Conjugate  Foci   130 

31.  Images  forme  1  by  Convex  Lenses   131 

32.  Magnifying  Property  of  Convex  Lenses   132 

33.  Diminishing  Effect  of  Concave  Lenses   ib. 


xiii 


Xiv  LIST  OF  ILLUSTRATIONS. 

34.  Cannon  of  the  Palais  Royal     .                               .  134 

35.  FresnePs  Lighthouse  Apparatus  ,136 

36.  Lantern  of  a  First-Class  Lighthouse   140 

37.  The  Compound  Microscope                                     0  143 

38.  The  Theory  of  the  Compound  Microscope     ....  144 

39.  Photo-Electric  Microscope   147 

40.  Solar  Microscope                                                   .  148 

41.  The  Galilean  Telescope   155 

42.  The  Astronomical  Telescope   156 

43.  Section  of  an  Astronomical  Telescope   157 

44.  Section  of  the  Gregorian  Telescope   160 

45.  Gregorian  Telescope   161 

46.  Section  oi  a  Newtonian  Telescope     .......  162 

47.  Herschellian  Telescope   164 

48.  Foucault's  Large  Telescope   163 

49.  Foucault's  Small  Telescope   171 

50.  Section  of  the  Magic  Lantern   179 

51.  Magic  Lantern   182 

52.  The  Phantasmagoria   184 

53.  The  Phantascope   185 

54.  Phantasmagoria  (Robertson)   194 

55.  Wizard  Dance     .    .   198 

56.  Nostradamus  and  Marie  de  Medicis                           .  201 

57.  The  Arrangement  of  the  Reversing  Prism      ....  233 

58.  The  Goat  Trick   205 

59.  How  to  see  through  a  Brick   207 

60.  The  Polemoscope   210 

61.  Protection  against  ill-natured  People   213 

62  .   218 

63.  Anamorphosis   220 

64.  Effect  of  Cat  Paper-work   225 

65.  Seditious  Toys   229 

66.  Diorama   234 

67   237 

68.  Stereoscope                                                           .  238 

69.  The  Principle  of  the  Refracting  Stereoscope  ....  239 

70.  The  Camera  Obscura  .#   243 

71.  Section  of  Camera  Lucida   247 

72.  The  Spectre— an  Optical  Illusion   269 

73.  How  to  produce  Spectres   271 


THE  WONDERS  OF  OPTICS. 


PART  I. 

THE  PHENOMENA  OF  VISION. 


CHAPTER  I. 

THE  EYE. 

The  Eye  is  at  once  the  most  wonderful  and  the  most 
useful  of  all  our  organs  of  sense.  It  is  especially  by 
means  of  the  eye  that  we  gain  a  knowledge  of  the  ex- 
terior world.  Our  other  senses  are  far  more  limited  in 
their  action :  thus  the  sense  of  touch  only  extends  to 
objects  within  our  reach  ;  the  sense  of  taste  is  only  a 
delicate  and  exquisite  modification  of  the  sense  of  touch  ; 
the  sense  of  smell  can  only  be  exercised  on  substances 
that  are  close  to  us ;  and  the  use  of  our  ears  is  limited 
by  the  distance  at  which  the  loudest  sound  ceases  to 
impress  them.  But  the  eye  has  the  privilege  of  extend- 
ing its  dominion,  whether  for  mere  enjoyment  or  for 
serious  instruction,  far  beyond  the  limits  of  this  little 
world.    Not  only  is  it  the  origin  of  all  our  ideas  upon 

15 


16 


THE  WONDERS  OF  OPTICS. 


every  object  that  comes  within  its  ken;  not  only  does 
it  reveal  to  us  our  own  position  and  that  of  our  sur- 
roundings ;  but,  thanks  to  the  discoveries  of  modern 
science,  it  is  able  to  admire,  on  the  one  hand,  a  worlcl 
of  infinite  minuteness  that  remained  unknown  to  us  for 
centuries,  and,  on  the  other,  the  immeasurable  immen- 
sity of  the  starry  universe. 

Admirable  as  the  eye  undoubtedly  is  through  the 
possession  of  the  power  of  vision,  it  is  also  capable  of 
enchanting  us  by  its  own  particular  beauties.  Not  to 
speak  of  its  internal  mechanism,  which  we  shall  consi- 
der very  fully  by  and  by,  let  us  for  a  moment  examine 
its  outward  appearance.  Have  you  never,  dear  reader, 
been  enchanted  with  a  pair  of  soft  and  gentle  eyes,  or 
with  a  couple  of  black  orbs  veiled  with  long  dark  lashes, 
or  with  those  wondrous  eyes  that  rival  the  heavens  in 
colour  and  depth,  shedding  on  you  rays  of  light  whose 
mute  eloquence  was  irresistible?  If  it  be  true  that 
man's  face  is  the  canvas  upon  which  the  affections  and 
desires  of  his  mi  id  are  depicted  as  soon  as  they  are 
formed,  the  eyes  are  unquestionably  the  central  point 
of  the  picture,  and  it  is  in  them,  as  in  a  looking-glass, 
that  every  sentiment  that  passes  across  our  brain  is  re- 
flected. 

When  the  mind  is  undisturbed,  says  Buffon,  all  the 
parts  of  the  face  are  in  a  state  of  repose ;  their  pro- 
portion, unity,  and  general  appearance  indicate  the 
pleasing  harmony  of  our  thoughts  and  the  perfect  calm- 
ness of  our  mind ;  but  when  we  are  agitated,  the  human 
face  becomes  a  living  picture,  in  which  the  passions 
that  disturb  us  are  depicted  with  equal  force  and 
delicacy,  a  picture  in  which  every  emotion  is  expressed 
by  a  stroke,  every  action  by  a  letter,  so  to  speak ;  in 
which  the  quickness  of  the  impression  outstrips  the  will, 
and  reveals  by  the  most  sympathetic  signs  the  image  of 
our  secret  trouble. 


THE  EYE. 


17 


It  is  more  especially  in  the  eyes,  adds  this  great 
naturalist,  that  these  signs  are  manifested  and  recog- 
nised. The  eye  is  connected  with  the  mind  more  than 
any  other  organ :  it  seems  almost  to  be  in  contact  with 
it  and  to  participate  in  all  its  movements  ;  it  expresses 
in  obedience  to  it  the  strongest  passions  and  the  most 
tumultuous  emotions,  as  well  as  the  gentlest  thoughts 
and  most  delicate  sentiments,  and  reproduces  them  in 
all  their  force  and  purity  just  as  they  have  sprung  into 
existence ;  it  transmits  them  with  exquisite  rapidity 
even  to  the  minds  of  others,  where  they  once  more  be- 
come impressed  with  all  their  original  fire,  movement, 
and  reality.  The  eye  both  receives  and  reflects  the 
light  of  thought  and  the  warmth  of  sentiment,  and  is 
at  once  the  sense  of  the  mind  and  the  tongue  of  the 
intellect.  Persons  who  are  short-sighted,  or  who  sqaint, 
have  much  less  of  this  external  intelligence  that  dwells 
in  the  eye.  It  is  only  the  stronger  passions  that  can 
bring  the  other  features  of  the  face  into  play,  that  are 
depicted  on  their  physiognomy;  and  the  effects  of  fine 
thought  and  delicate  feeling  are  rendered  apparent  with 
much  greater  difficulty. 

The  elegant  author  of  L'Histoire  Naturelle  rightly 
thinks  that  we  are  so  accustomed  only  to  see  tilings 
from  the  outside,  that  we  are  hardly  aware  how  much 
this  exterior  view  of  everything  influences  the  judgment 
of  even  the  gravest  and  most  thoughtful  of  us.  Thus 
we  are  apt  to  set  down  a  man  as  unintellectual  whose 
physiognomy  does  not  particularly  strike  us ;  and  we 
iillow  his  clothes,  and  even  the  manner  in  which  he  wears 
his  hair,  to  influence  our  judgment  of  him.  Hence,  our 
author  goes  on  to  say,  not  wholly  without  some  show 
of  reason,  that  a  man  of  sense  ought  to  look  upon  his 
clothes  as  part  of  himself,  because  they  really  are  so  in 
the  eyes  of  others,  and  play  an  important  part  in  the 
general  idea  that  is  formed  of  him  who  wears  them 

B 


18 


THE  WONDERS  OF  OPTICS. 


The  vivacity  or  languor  of  the  movement  of  the  eyes 
forms  one  of  the  chief  characteristics  of  facial  expression, 
and  their  colour  helps  to  render  this  characteristic  more 
striking.  The  different  colours  seen  in  the  eye  are  dark 
hazel,  or  black,  as  it  is  generally  called,  light  hazel,  blue, 
greenish  grey,  dark  grey,  and  light  grey.  The  velvety 
substance  which  gives  the  colour  to  the  iris  is  arranged 
in  little  ramifications  and  specks,  the  former  being  di- 
rected towards  the  centre  of  the  eye,  the  latter  filling  up 
the  gaps  between  the  threads.  Sometimes  they  are  both 
arranged  in  so  regular  a  manner  that  instances  have 
been  known  in  which  the  irises  of  different  eyes  have 
appeared  to  be  so  much  alike  that  they  seemed  to  have 
been  copied  from  the  same  design.  These  little  threads 
and  specks  are  held  together  by  a  very  fine  network. 

The  commonest  colours  seen  in  the  eye  are  hazel  and 
blue,  and  it  mostly  happens  that  both  these  colours  are 
found  in  the  same  individual,  giving  rise  to  that  peculiar 
greenish-grey  hue  that  is  far  from  being  uncommon. 
Buffon  thinks  that  blue  and  black  eyes  are  the  most 
beautiful,  but  this  of  course  is  a  matter  of  taste.  It  is 
true  that  the  vivacity  and  fire  which  play  so  important 
a  part  in  giving  character  to  the  eye,  are  more  percep- 
tible in  dark  eyes  than  in  those  whose  tints  are  lighter  ; 
black  eyes,  therefore,  have  greater  force  of  expression, 
while  in  blue  eyes  there  is  more  softness  and  delicacy. 
In  the  former  we  see  a  brilliant  fire,  which  sparkles 
uniformly  on  account  of  the  iris,  which  is  of  the  same 
colour  throughout,  giving  in  all  parts  the  same  reflection ; 
but  a  great  difference  may  be  perceived  in  the  intensity 
of  the  light  reflected  from  blue  eyes,  from  the  fact  of  the 
various  tints  of  colour  producing  different  reflections. 
There  are  some  eyes  that  are  remarkable  for  being  almost 
destitute  of  colour,  and  appear  to  be  constituted  in  an 
abnormal  manner.  The  iris  is  tinted  with  shades  of 
blue  and  grey  of  so  light  a  hue  that  it  appears  quite 


THE  EYE. 


19 


white  in  some  places.  The  shades  of  hazel  in  such  eyes 
are  so  light  that  they  are  hardly  distinguishable  from 
grey  and  white,  in  spite  even  of  the  contrast  of  colour. 

For  our  part,  we  think  that  the  beauty  of  the  eye 
consists  not  so  much  in  its  colour,  or  even  in  its  har- 
mony with  the  rest  of  the  face,  but  in  its  expression. 

There  are  also  numerous  instances  of  green  eyes. 
This  colour  is,  of  course,  much  less  frequent  than  blue, 
grey,  or  hazel.  It  often  happens,  too,  that  the  two 
eyes  vary  in  colour  in  the  same  individual.  This  defect 
is  not  confined  to  the  human  species,  being  shared  by 
the  horse  and  the  cat.  In  most  other  animals  the  co- 
lour of  the  two  eyes  is  always  similar.  The  colour  of 
the  eye  in  most  animals  is  either  hazel  or  grey.  Aristotle 
imagined  that  grey  eyes  were  stronger  than  blue,  that 
those  persons  whose  eyes  are  prominent  cannot  see  so 
far  as  others,  and  that  brown  eyes  are  less  valuable  in 
the  dark  than  those  of  another  tint ;  but  modern  inves- 
tigations have  failed  to  bear  out  the  ancient  philoso- 
pher's ideas  with  regard  to  the  human  eye. 

Although  the  eye  appears  to  move  about  in  every 
direction,  it  has  in  reality  only  one  movement,  that  of 
rotation  round  its  centre,  by  means  of  which  the  eye- 
ball rises  or  falls,  or  passes  from  side  to  side  at  will. 
In  man  the  eyes  are  parallel  with  each  other  in  relation 
to  their  axes  ;  he  can  consequently  direct  them  at  plea- 
sure upon  the  same  object :  but  in  most  animals  this 
parallelism  is  wanting.  In  some  cases  the  eyes  of  ani- 
mals are  set  almost  back  to  back,  rendering  it  impossi- 
ble for  them  to  see  the  same  object  with  both  eyes  at 
once. 

Buffon  makes  the  remark,  that  after  the  eyes,  the 
eyebrows  contribute  more  strongly  than  any  other  part 
of  the  face  towards  giving  character  to  the  physiognomy, 
being,  inasmuch  as  they  differ  in  their  nature  from  the 
other  features,  more  apparent  by  contrast,  and  hence 


20 


THE  WONDERS  OF  OPTICS. 


strike  us  more  than  any  other  portion  of  the  counte- 
nance. They  are,  in  fact,  a  shadow  in  the  picture, 
bringing  its  colour  and  drawing  into  strong  relief.  The 
eyelashes  also  contribute  their  effect ;  when  they  are 
long  and  thick,  they  overshadow  the  eye,  making  its 
glance  appear  softer  and  more  beautiful.  The  ape  is 
the  only  other  animal  besides  man  that  possesses  two 
eyelashes,  the  rest  having  them  only  on  the  upper  eye- 
lid. Even  in  man  they  are  more  abundant  in  the  upper 
eyelid  than  in  the  lower.  The  eyebrows  have  but  two 
movements,  upward  and  downward,  governed  by  the 
muscles  of  the  forehead.  In  the  action  of  frowning  we 
not  only  lower  them,  but  move  them  slightly  towards 
each  other.  The  eyelids  serve  to  protect  the  eyeball, 
and  keep  the  cornea  from  becoming  dry.  The  upper 
eyelid  has  the  power  of  raising  and  lowering  itself,  the 
lower  one  being  almost  destitute  of  movement.  Although 
the  motion  of  the  eyelids  is  an  effort  of  will,  there  are 
times  when  it  is  impossible  to  keep  them  open,  as  for 
instance  when  we  are  overpowered  by  sleep,  or  when 
the  eyes  are  suddenly  subjected  to  the  effects  of  strong 
light.  The  eyelid  is  a  most  admirable  arrangement  for 
the  protection  of  the  eye,  and  it  is  almost  impossible  to 
admire  this  provision  of  nature  too  much,  even  when 
we  confine  ourselves  to  an  outward  examination  of  it. 
It  is  not  merely  the  outward  mechanism  and  motion  of 
the  eyelids,  nor  the  colour  of  the  eyes,  that  constitutes 
their  beauty ;  we  have  already  said  that  the  leading 
characteristic  of  the  eye  was  expression.  It  is  this  ex- 
pression which  causes  the  eye  to  appear  to  speak,  to  fire 
up  suddenly,  to  sparkle  with  flashes  of  light,  to  languish 
or  conceal  itself  underneath  its  lashes,  to  raise  itself 
with  inspiration,  or  to  pierce  the  abyss  of  thought,  just 
according  to  the  particular  sentiment  governing  the 
mind  at  the  moment.  Hence  it  is  expression  that  con- 
stitutes the  true  beauty  of  the  eye :  every  one  knows 


THE  EYE. 


21 


instances  of  eyes  which,  while  at  rest,  would  never  be 
noticed  by  anybody,  but  which,  when  once  animated  by 
intense  eloquence,  lend  to  the  voice  of  their  possessor 
an  unexpected  power,  which  moves  and  transports  the 
listener  to  an  extent  infinitely  beyond  that  resulting 
from  the  simple  spoken  words. 

Enough,  however,  has  been  said  upon  the  external 
aspect  of  the  human  eye ;  we  will,  therefore,  at  once 
endeavour  to  penetrate  the  circle  in  which  are  con- 
tained the  wonders  that  this  little  book  is  intended  to 
describe.  The  object  of  these  lines  is  not  so  much  to 
describe  the  beauty  of  man's  glances,  nor  the  value  of 
his  senses,  but  rather  to  make  known  those  illusions  to 
which  the  most  sagacious  of  all  his  senses  is  apt  to  fall 
a  prey.  But  before  entering  the  temple  it  was  but 
right  to  have  bestowed  a  little  admiration  upon  the 
fa§ade.  By  the  way,  as  we  are  about  to  describe  many 
illusory  wonders,  clo  not  let  us  commence  by  deceiving 
ourselves  with  regard  to  our  first  marvel — the  eye  itself. 
A  great  philosopher  calls  the  eyes  the  windows  of  the 
soul,  and,  although  meant  as  a  poetical  image,  the  say- 
ing is  not  far  from  the  truth  ;  for  the  optic  nerve  by 
which  we  see  external  objects,  is  an  extension  of  the 
nerves  of  the  brain,  whose  functions  and  actions  are 
an  unfathomable  mystery. 


\ 


22 


THE  WONDERS  OF  OPTICS. 


CHAPTER  II. 

THE  STRUCTURE  OF  THE  EYE. 

Of  all  the  senses,  says  an  ardent  admirer  of  nature, 
the  sight  is  certainly  that  which  furnishes  the  mind  with 
the  quickest  and  most  widely-extended  perceptions.  It 
is  the  source  of  the  richest  treasures  of  the  imagination, 
and  of  our  ideas  of  the  beauty,  order,  and  unity  of  the 
world  around  us.  How  unhappy  are  those  whom  a 
hard  fate  has  deprived  of  the  sense  of  sight  from  their 
birth !  Alas !  the  finest  day  and  the  darkest  night 
differ  in  nothing  as  far  as  they  are  concerned  ;  the  light 
of  heaven  never  brings  joy  into  their  hearts.  The 
enamelled  beauties  of  a  bed  of  flowers,  the  varied  plu- 
mage of  the  peacock,  the  glories  of  the  rainbow  are 
alike  unknown  to  them.  They  cannot  contemplate  from 
the  mountain  height  the  beauties  of  the  valley  beneath  ; 
the  fields  golden  with  the  harvest,  the  meadows  smiling 
with  verdure,  and  watered  by  winding  rivers,  and  the 
habitations  of  man  dotted  about  here  and  there  over 
the  surface  of  this  magnificent  picture.  To  them  is 
unknown  the  sight  of  the  mighty  ocean ;  and  the 
innumerable  legions  of  the  cloud  army  of  Heaven  are 
to  them  as  if  they  did  not  exist.  The  impenetrable 
obscurity  which  surrounds  them  allows  them  neither  the 
contemplation  of  what  is  grandest  in  man's  outward 
aspect,  nor  even  the  admiration  of  those  qualities  which 
they  themselves  would  hold  most  dear. 


THE  STRUCTURE  OF  THE  EYE. 


23 


A  strong  sentiment  of  pity  should,  therefore,  ani- 
mate the  breast  of  every  right-thinking  man,  when  he 
considers  the  unhappy  condition  of  those  who  are  born 
blind. 

The  eye  infinitely  surpasses  in  its  complexity  and 
beauty  of  structure  all  the  other  organs  of  sense,  and 
is  most  unquestionably  the  most  marvellous  object  that; 
the  human  mind  is  capable  of  examining  and  under, 
standing,  Let  us  first  examine  the  external  parts  of 
this  wonderful  organ.  With  what  a  singular  system  of 
entrenchments  and  defences  do  we  find  the  eye  pro- 
vided !  It  is  itself  placed  in  the  head  at  a  certain 
depth,  and  surrounded  on  all  sides  by  solid  bone,  so  that 
it  is  only  with  the  greatest  difficulty  that  it  is  hurt  by 
accident  from  without."  The  eyebrows  also  play  their 
part  as  protection  to  the  eye,  and  prevent  the  perspira- 
tion from  entering  and  irritating  the  organ.  The  eye- 
lids too  are  always  ready  to  rush  to  the  rescue,  whether 
to  protect  the  eye  from  outward  attacks,  or  to  shade  it 
from  too  strong  a  light  during  sleep.  The  eyelashes 
not  only  add  to  the  beauty  of  the  eye,  but  they  shade 
it  from  the  too  brilliant  light  of  the  sun,  and  act  as 
advanced  guards  to  prevent  thje  entrance  of  dust  or  any 
other  foreign  body  with  which  the  eyes  might  be  in- 
jured. 

But  its  internal  structure  is  still  more  admirable. 
The  globe  of  the  eye  is  almost  spherical  and  measures 
nearly  one  inch  in  diameter.  Fig.  1  is  a  view  of  the 
eyeball,  showing  the  details  of  its  structure  ;  the  various 
membranes  surrounding  it  have  been  cut  away  in  order 
that  it  may  be  better  examined.  If  we  commence  our 
examination  by  the  exterior  portion  of  the  front,  we 
shall  first  find  immediately  beneath  the  eyelashes  a  per- 
fectly transparent  membrane  (c),  called  the  cornea.  It 
is  a  prolongation  of  the  hard  opaque  external  coating 
of  the  eye,  called  the  sclerotic  membrane,  and  marked 
s  in  the  figure.    The  cornea  is  sufficiently  hard  in  its 


THE  WONDERS  OF  OPTICS. 


nature  to  present  a  strong  resistance  to  any  violence 
from  without. 

Immediately  beneath  the  cornea  and  in  contact  with 
it  is  the  aqueous  humour,  a  thin  transparent  liquid  oc- 
cupying a  small  portion  of  the  front  of  the  eye. 

Next  comes  the  iris,  a  circular  disc  perforated  with  a 
round  hole  in  the  middle,  and  coloured  with  various 
shades  of  blue,  brown,  and  grey. 

The  opening  in  the  centre,  which  appears  like  a  black 
spot  when  the  eye  is  examined,  is  not  really  an  object, 
but  simply  an  aperture,  capable  of  changing  its  size 
according  to  the  quantity  of  light  striking  the  eye. 
This  change  of  size  in  the  opening,  or  pupil,  as  it  is 
popularly  called,  is  effected  by  the  contraction  or  ex- 


I 


Fig.  1.— Section  of  the  Eye. 

pansion  of  the  iris,  which  thus  possesses  the  peculiar 
property  of  exactly  proportioning  the  amount  of  'light 
that  enters  the  eye,  so  that  there  is  never  too  much  or 
too  little.  It  is  through  the  pupil  that  the  rays  of 
li^ht  proceeding  from  the  various  objects  around  us  pass 
into  the  interior  of  the  eye,  and  form  an  image  upon 
the  retina,  as  will  be  afterwards  explained. 

Immediately  behind  the  pupil  is  0,  a  bi-convex  lens 
to  transmit  the  rays  of  light  to  the  retina.  It  isgener* 
ally  called  the  crystalline  lens. 


THE  STRUCTURE  OF  THE  EYE. 


25 


From  the  crystalline  lens  to  the  back  of  the  eyeball, 
ia  a  space  more  or  less  globular  in  form,  containing 
a  gelatinous  diaphanous  mass  somewhat  resembling 
white  of  egg  in  appearance,  and  called  the  vitreous 
humour. 

Behind  the  vitreous  humour,  and  immediately  oppo- 
site the  pupil  and  lens,  is  the  most  delicate  and  im- 
portant of  all  the  membranes  of  the  eye,  the  retina, 
which  serves  as  a  screen  whereon  are  received  the 
images  of  the  objects  around  us.  This  membrane  is  an 
expansion  of  the  optic  nerve  N  leading  from  the  brain, 
and  lines  the  whole  of  the  interior  of  the  eye.  The 
eye  is  also  enveloped  in  a  second  membrane  (c),  called 
the  choroid,  which  is  impregnated  with  a  black  pigment. 
Round  this  is  wrapped  a  third  membrane,  the  sclerotic 
(s),  which  unites  with  the  cornea  in  front  of  the  eyeball. 

The  crystalline  lens  through  which  all  the  rays  pass 
before  they  reach  the  retina,  possesses  the  marvellous 
power  of  being  able  to  modify  its  curvature  in  such  a 
manner  as  to  adapt  itself  to  the  distance  of  the  object 
seen,  and  thus  throw  a  distinct  image  on  the  retina. 
When  we  come  to  talk  of  the  properties  of  lenses,  we 
shall  see  that  the  focus  of  a  lens  differs  for  objects  at 
different  distances ;  if,  therefore.,  the  eye  were  not  pro- 
vided with  some  such  means  for  altering  the  focus  of  the 
crystalline  lens,  we  should  only  see  objects  distinctly  at 
one  particular  point.  The  crystalline  lens  consists  of 
infinite  numbers  of  extremely  thin  transparent  little 
plates,  each  of  which  is  in  itself  composed  of  fine  fibres 
so  united  together  as  to  be  capable  of  a  small  degree  of 
compression  or  extension.  Hence  the  power  of  the 
lens  to  alter  its  form  according  to  circumstances.  It  is 
calculated  that  the  human  eye  contains  over  five  mil- 
lions of  the  laminae  above  referred  to.  With  such  won- 
ders is  the  world  of  nature  replete, — wonders  that  we 
daily  and  hourly  pass  by  without  examination. 


26 


THE  WONDERS  OF  OPTICS. 


It  is  by  means  of  this  ingenious  and  inimitable  struc- 
ture of  the  eye  that  external  objects  pass  from  the  do- 
main of  the  material  world  into  that  of  the  mind,  and 
become  accessible  to  every  faculty  of  our  brain.  Of  its 
own  accord,  arid  without  apparently  any  effort  of  our 
own  will,  does  this  marvellous  mechanism  adapt  itself 
to  all  the  variations  of  distance  and  intensity  of  light, 
a  power  possessed  by  no  instrument  as  yet  constructed 
by  the  hand  of  man — being  capable,  as  it  is,  of  distin- 
guishing instantaneously  between  the  distance  of  the 
remotest  nebulae  and  that  of  the  letters  forming  this 
page.  This  wonderful  organ,  writes  Brewster,  may  be 
considered  as  being  the  sentinel  that  guards  the  passage 
between  the  world  of  matter  and  that  of  mind,  and  as 
the  medium  through  which  they  interchange  all  their 
communications.  The  optic  nerve  perceives  the  objects 
written  on  the  retina  by  the  hand  of  natur.e,  and  con- 
veys them  to  the  brain  in  all  their  integrity  of  form  and 
colour. 

The  path  of  the  rays  of  light  and  the  formation  of 
images  upon  the  retina  are  shown  in  the  preceding 
figure.  At  first  sight  it  will  be  perceived  that  the  ob- 
jects thereon  depicted  are  in  a  reversed  position,  that  is 
to  say,  when  we  look  at  a  view  similar  to  that  shown  in 
fig.  2,  we  should  find,  if  we  had  any  means  of  observing 
the  positions  of  objects  reflected  on  our  retina,  that  the 
flock  of  sheep  coming  up  the  road  were  at  the  top  oi  the 
eye,  while  the  trees,  the  roof  of  the  house,  and  the  chim- 
ney were  in  the  contrary  position.  Similar  reversed 
images  may  be  seen  in  dark  rooms,  by  holding  a  screen 
before  any  little  crack  or  pinhole  in  the  door  or  shutter 
of  the  room.  In  fig.  2  the  keyhole  of  the  door  is  repre- 
sented as  playing  the  part  of  a  lens.  The  author,  in 
common  with  almost  every  other  boy,  observed  this  fact 
at  a  very  early  age,  and  the  idea  immediately  struck 
him  that  it  would  be  only  necessary  to  fix  these  images 


THE  STRUCTURE  OF  THE  EYE. 


27 


to  procure  exact  representations  of  natural  scenery  ;  but 
in  making  inquiries  into  the  subject,  he  found  that  his 
juvenile  observations  had  been  made  a  little  too  late, 
photography  having  already  gained  the  end  he  intended 
striving  for. 


Fig.  2.-  A  Camera  Obscura. 


Seeing  that  the  images  of  all  objects  appear  on  our 
retina  upside  down,  the  student  is  naturally  disposed  to 
ask  how  it  happens  that  we  do  not  see  them  in  that 
position.  Physiologists  and  natural  philosophers  have 
advanced  numerous  theories  on  the  subject.  Some, 
with  Buffon,  admit  at  once  that  it  is  by  habit  and  edu- 
cation of  the  eye  that  we  see  objects  unreversed. 
Others,  like  the  great  physiologist  Miiller,  imagine  that 
as  we  see  everything  upside  down,  and  not  a  single  ob- 
ject only,  we  have  no  points  of  comparison,  and  practi- 
cally ignore  the  reversal.  The  truth,  however,  appears 
to  be  that  it  is  the  brain,  and  not  the  eye,  that  possesses 
the  power  of  determining  the  real  position  of  what  we 
see.  That  the  eye  alone  has  no  power  of  determining 
the  positions  of  objects  by  itself,  may  be  easily  proved 
by  showing  a  person  an  astronomical  object,  such  as  the 
moon  through  a  telescope.    Unless  the  observer  has 


28 


THE  WONDERS  OF  OPTICS. 


been  already  familiarized  with  the  appearance  of  our 
satellite,  he  will  not  know  whether  the  image  he  sees  is 
reversed  or  not.  It  is  the  brain,  therefore,  and  the 
brain  only,  that  has  the  power  of  determining  the  posi- 
tion of  objects  around  us,  without  taking  into  considera- 
tion the  reversed  picture  of  them  that  is  depicted  on  our 
retina.  The  student  who  takes  an  interest  in  the  struc- 
ture of  this  important  organ,  would  do  well  to  procure 
a  sheep's  or  bullock's  eye  from  the  butchers,  and  dissect 
it  carefully  with  a  sharp  penknife  and  pair  of  scissors. 
The  image  formed  on  the  retina  may  be  easily  seen  by 
cutting  away  the  sclerotic  and  choroid  coatings  at  the 
back  of  the  eye. 

The  ordinary  distance  of  distinct  vision  for  small 
objects,  such  as  the  letters  of  a  book,  is  from  ten  to 
twelve  inches.  But  possibly  there  do  not  exist  two 
pairs  of  eyes  in  the  world  whose  foci  are  the  same. 
Even  in  the  same  individual  it  frequently  happens  that 
the  focal  length  of  the  eyes  differs  considerably.  In 
some  persons  the  focus  of  the  eye  is  so  reduced  that 
they  are  obliged  to  bring  the  object  they  are  examining 
within  six,  and  even  four  inches  of  their  eyes,  before 
they  can  see  it.  This  defect  is  known  ordinarily  as 
short  sight,  and  results  from  the  too  great  convexity  of 
the  cornea  and  crystalline  lens.  It  is  corrected  by 
wearing  spectacles  with  concave  glasses.  Others  again, 
on  the  contrary,  place  the  book  or  object  they  are  look- 
ing at,  at  a  greater  distance  from  the  eye  than  that 
named.  Such  people  are  called  long-sighted,  and  the 
defect  results  from  the  too  great  flatness  of  the  cornea 
and  the  crystalline  lens.  The  fault  is  of  course  cor- 
rected by  the  use  of  spectacles  containing  convex 
lenses. 

Long-sightedness  is  generally  the  result  of  old  age, 
and  it  may  be  taken  as  a  fact  that  the  older  we  grow 
the  flatter  becomes  the  crystalline  lens.    Hence  short- 


THE  STRUCTURE  OF  THE  Em 


29 


sighted  people  have  been  known  to  recover  their  sight 
perfectly  as  they  advance  in  years  through  the  natural 
process  of  the  flattening  of  the  crystalline  lens.  These 
matters,  however,  will  be  more  fully  treated  of  when 
we  begin  to  speak  of  the  properties  of  lenses  of  different 
forms  and  curvature** 


30 


THE  WONDERS  OF  OPTICS. 


CHAPTER  111. 

THE  ERRORS  OF  THE  EYE. 

It  is  with  our  own  organization  that  we  shall  com- 
mence our  task  of  exposing  the  illusions  that  we  shall 
meet  with  during  our  optical  experiments, — in  fact  with 
that  wonderful  and  important  organ  of  our  body  that 
we  are  apt  to  look  upon  as  sure  and  infallible,  but 
which  we  shall  find  is  deceiving  us  constantly,  and 
hourly  proving  the  fallacy  of  the  popular  saying,  that 
"  every  one  must  believe  his  own  eyes."  In  ancient 
times  there  existed  a  school  of  sceptics  who  doubted 
everything  beginning  with  Pyrrho,  the  great  theorist, 
and  ending  with  the  follower  of  his  school  who  doubted 
the  existence  of  muscular  force  even  after  he  had  re- 
ceived a  sound  box  on  the  ear  from  an  opponent  of  his 
system  of  philosophy.  If  any  of  our  readers  were  to 
became  followers  of  Pyrrho,  they  might  easily  do  so 
when  considering  the  numberless  illusions  we  shall 
describe  to  them,  if  they  did  not  remember  that  if  our 
senses  are  subject  to  error,  we  have  a  brain  to  set  them 
right :  our  mind,  if  logical  and  well  regulated,  soon 
discovers  errors  of  observation,  and  speedily  places  our 
judgment  on  the  most  solid  basis.  We  shall  find  end- 
less instances  of  this  throughout  our  little  book.  If 
we  are  dazzled  with  illusions  from  time  to  time  we  shall 
as  often  recover  ourselves;  and  no  matter  how  beauti- 
ful or  interesting  these  deceptions  may  appear,  we  shall 
speedily  be  able  to  convince  ourselves  that  they  are 


THE  ERRORS  OF  THE  EYE. 


31 


unreal.  In  this  chapter  we  shall  only  speak  of  those 
errors  of  the  eye  of  which  we  have  actually  lost  all 
cognizance,  so  effectually  has  our  judgment  succeeded 
in  counteracting  their  influence. 

We  all  know  that  the  first  thing  a  child  does  with  its 
eyes,  even  when  it  is  only  five  or  six  weeks  old,  is  to 
turn  them  towards  the  most  brilliant  object  within  its 
reach.  Instinctively  and  without  being  aware  of  it, 
the  child's  eye  seems  to  seek  the  light.  The  whole  of 
nature,  from  the  lowest  plant  to  the  baby  in  the  cradle, 
appears  more  or  less  endowed  with  this  instinct  of  turn- 
ing towards  th^j  light. 

From  the  time  that  children  begin  to  distinguish  ob- 
jects, their  eyes  are  liable  to  be  affected  by  two  causes 
of  error.  Before  being  able  to  judge  of  the  position  of 
things  surrounding  them,  they  see  everything  upside 
down  ;  they  consequently  acquire  a  false  impression  of 
the  position  of  obj-ects.  The  next  cause  of  error  that  is 
likely  to  mislead  them  is  the  fact  of  their  seeing  every- 
thing double,  a  separate  image  of  everything  being 
formed  on  each  eye ;  and  it  can  only  be  by  the  experi- 
ence gained  through  the  sense  of  touch  that  they  can 
acquire  the  knowledge  necessary  to  rectify  these  er- 
rors, and  see  those  objects  single  which  appear  to  them 
double.  This  error  of  sight,  as  well  as  the  first  one,  is 
s*-t  right  so  easily  in  the  end,  that  although  in  reality 
we  see  everything  double  and  upside  down,  we  imagine 
that  we  see  them  single,  and  in  their  proper  positions, 

state  of  things  brought  about  entirely  through  ano- 
ther sense  exercising  its  power  over  our  judgment ;  and 
it  is  hardly  too  much  to  say  that,  if  that  sense  were  de- 
prived of  the  power  of  feeling,  our  eyes  would  deceive 
us,  not  only  as  to  the  number,  but  the  position  of  the 
objects  within  our  view. 

It  is  very  easy  to  convince  ourselves  that  we  really 
see  objects  double,  although  we  imagine  them  to  be  only 


32 


THE  WONDERS  OF  OPTICS. 


single.  We  have  only  to  look  at  the  same  object  first 
with  the  right  eye,  and  we  shall  see  it  directly  against 
some  portion  of  the  wall  of  the  room  in  which  we  are 
sitting  ;  then  looking  at  it  with  the  left  eye,  we  shall 
see  that  it  covers  a  different  part  of  the  wall.  This  ex- 
periment is  easily  tried,  and  is  very  convincing.  Thus 
we  see  that  an  image  is  formed  on  both  eyes,  and  wTe 
consequently  see  the  object,  whatever  it  may  be,  re- 
peated twice.  By  degrees,  however,  the  eyes  gain  the 
power  of  converging  their  axes  on  objects  at  different 
distances,  so  that  they  fall  on  similar  portions  of  each 
retina,  and  so  convey  a  single  impression  to  the  brain. 
Thus,  for  instance,  if  we  look  at  a  pencil  held  up  at 
arm's  length,  and  then,  without  changing  the  position 
of  the  eyeball,  look  at  some  distant  object,  we  shall  see 
it  double.  Let  us,  however,  converge  the  eyes  upon  it, 
and  the  two  images  unite.  Reverse  the  experiment  by 
now  looking  at  the  pencil  without  converging  the  eyes 
upon  it,  and  we  shall  see  that  object  double  in  its  turn. 
The  same  thing  happens  if  we  push  aside  one  of  the 
eyes  with  the  finger  while  looking  at  any  object. 
During  severe  illness  it  often  happens  that  the  patient 
from  extreme  weakness  loses  the  power  of  convergence, 
and  consequently  sees  every  thing  double,  and  we  con- 
tinually see  children's  faces  wearing  a  most  distressing 
appearance  through  having  temporarily  lost  the  power 
of  moving  the  muscles  of  the  eye.  It  is  a  common  ex- 
pression to  use  in  speaking  of  drunken  people,  that  they 
see  double,  but  the  saying,  unlike  many  others,  is  no 
metaphor ;  when  a  man  gets  drunk  he  loses  his  power 
over  the  muscles  of  his  eye,  just  as  he  does  over  those 
that  sustain  his  body,  and  the  instinctive  closing  of  one 
eyelid,  in  order  that  he  may  see  objects  single,  is  an 
effort  of  his  weakened  judgment  to  set  things  right  once 
more. 

While  on  this  subject  we  may  mention  the  experiment 


THE  ERRORS  OF  THE  EYE. 


33 


made  by  the  famous  English  surgeon  Cheselden  upon  a 
boy  who  was  born  blind,  and  upon  whom  he  operated 
successfully. 

This  boy,  who  was  thirteen  years  old  at  the  time  that 
Cheselden  restored  to  him  the  sense  of  sight,  was  not 
born  absolutely  blind,  his  affliction  having  been  caused 
by  a  cataract  or  film  spread  over  the  eyeball,  which  al- 
lowed him  to  distinguish  night  from  day,  or  black  from 
white  or  scarlet  when  placed  in  a  very  good  light, 
although  he  was  unable  to  perceive  the  form  of  things 
around  him.  At  first  Cheselden  operated  on  a  single 
eye,  perfectly  restoring  its  power  ;  but  so  little  idea  of 
distance  did  the  new  sense  convey  to  the  boy's  mind 
that  for  a  long  time  he  imagined  that  everything  touched 
his  eyeball,  just  as  those  he  felt  touched  his  skin,  and 
it  was  only  by  the  sense  of  touch  that  he  could  persuade 
himself  of  £he  fallacy  of  his  supposition.  At  first  he 
had  no  perception  of  form  whatever,  and  could  only 
recognize  objects  he  had  already  been  familiar  with 
after  he  had  felt  them  all  over.  He  was  a  long  time, 
for  instance,  before  he  could  distinguish  between  the 
dog  and  the  cat  without  touching  them,  and  was  greatly 
surprised  to  find  that  the  persons  and  things  he  had 
liked  best  when  blind  were  not  always  the  pleasantest 
to  his  newly  acquired  sense.  His  ideas  of  size,  too, 
were  all  at  fault,  and  he  could  not,  for  a  long  time,  be 
made  to  understand  how  his  father's  picture  could  be 
got  into  the  back  of  his  mother's  watch  ;  even  after  he 
had  possessed  his  sight  for  a  comparatively  long  time, 
he  could  still  only  recognise  people  he  had  known  during 
his  blindness  by  touching  their  faces.  Whenever  he 
saw  a  new  object  he  looked  at  it  attentively  for  some 
time,  in  order,  as  it  were,  to  learn  its  form  by  heart; 
but  his  memory  was  at  first  so  overtaxed  that  he  con- 
tinually forgot  his  visual  impressions,  and  mistook  one 
thing  for  another.    He  was  more  than  two  months  be- 


34 


THE   WONDERS  OF  OPTICS. 


fore  he  could  appreciate  form  as  depicted  in  a  painting 
or  drawing,  having  hitherto  learned  to  consider  pictures 
as  flat  objects.  When,  however,  he  began  to  understand 
the  power  of  light  and  shade  in  producing  the  repre- 
sentations of  solid  objects,  he  was  often  extremely  sur- 
prised to  find  the  surface  on  which  they  were  depicted 
quite  flat  when  he  touched  it.  The  same  thing  frequently 
happens  to  ourselves,  when  looking  at  the  photographs 
of  bas-reliefs  for  instance.  If  these  objects  be  well 
photographed,  with  the  proper  arrangement  of  light 
and  shade,  the  illusion  is  so  complete  that  the  finger 
involuntarily  touches  the  paper  to  feel  if  the  surface  is 
not  really  raised.  In  the  Bourse  at  Paris  there  are 
some  figures  painted  to  represent  bas-reliefs  in  so  won- 
derful a  manner,  that  numberless  bets  have  been  made, 
lost  and  won,  over  them.  When  feeling  such  repre- 
sentations of  solid  objects,  the  boy  would  often  ask 
those  around  him  wThich  of  his  senses  was  deceiving  him, 
his  sight  or  his  touch. 

At  first  he  saw  everything  of  an  enormous  size,  but 
as  he  saw  things  larger  than  those  around  him,  he  found 
the  latter  diminish.  He  ah o  imagined  that  there  was 
nothing  beyond  the  room  he  was  in,  and  could  not  be 
brought  to  comprehend  how7  the  house  could  be  larger. 
When  the  sight  of  the  second  eye  was  restored  to  him  a 
year  afterwards,  he  at  first  saw  ever}  object  of  an  enor- 
mous size,  just  as  in  the  case  of  the  first  eye;  but  as 
he  had  now  the  perfectly  educated  organ  to  help  him  as 
well  as  his  sense  of  touch,  he  soon  began  to  see  things 
under  their  natural  appearances. 

While  he  was  in  ignorance  of  what  sight  really  meant, 
he  was  not  particularly  anxious  to  undergo  the  opera- 
tion, saying  that  he  did  not  think  it  possible  to  derive 
more  pleasure  from  things  that  he  liked  than  he  did 
while  he  was  blind.  But  now  that  his  sight  was  restored 
he  found  every  fresh  object  a  new  pleasure.  When 


THE  ERRORS  OF  THE  EYE. 


35 


first  he  was  shown  the  landscape  from  the  top  of  a  high 
hill,  he  was  so  delighted  that  he  exclaimed  that  he  had 
found  another  sense.  When  his  second  eye  was  operated 
upon,  he  saw  things  apparently  twice  as  large  with  both 
eyes  as  with  the  one  already  restored  to  him.  Even  at 
first  he  seemed  to  have  no  difficulty  in  converging  the 
eyes  on  any  object. 

These  extracts  from  the  history  of  Cheselden's  patient 
show  us  how  utterly  incapable  the  eye  must  be  of 
rightly  understanding  the  number,  position,  size,  and 
form  of  objects  without  frequently  correcting  our  im- 
pressions by  the  aid  of  the  sense  of  touch. 


86 


THE  WONDERS  OF  OPTICS. 


CHAPTER  IV. 

OPTICAL  ILLUSIONS. 

Besides  the  errors  of  sight  already  spoken  of,  there 
are  other  illusions,  which  are  either  common  to  all  per- 
sons or  confined  to  certain  individuals,  the  knowledge 
of  which  will  serve  as  a  fitting  prelude  to  the  descrip- 
tions of  those  which  are  artificial. 

The  following  defecf,  for  instance,  is  one  which  is 
li  tie  known,  but  notwithstanding  our  ignorance  of  its 
existence  it  is  nevertheless  true  that  we  all  suffer  from 
it.  There  is  in  every  one's  eye  a  blind  spot,  totally  in- 
capable of  experiencing  the  effects  of  the  rays  of  light 
when  they  impinge  upon  it.  For  objects  situated  oppo- 
site to  this  particular  spot  we  are  as  completely  blind  as 
if  we  had  no  eyes  at  all.  To  convince  yourself  of  the 
truth  of  this  assertion  it  is  only  necessary  to  try  the 
following  simple  experiment. 

Place  upon  a  piece  of  white  paper  two  small  wafers, 
or  two  blots  of  ink  about  an  inch  and  a  half  apart. 
Take  the  sheet  in  your  right  hand,  and  hold  it  up  par- 
allel to  the  lines  of  the  eyes;  shut  the  left  eye,  and  fix 
the  right  eye  on  the  centre  of  the  left  wafer  or  ink-spot. 
Move  the  sheet  of  paper  steadily  towards  the  eye,  until 
it  is  about  two  inches  and  a  half  or  three  inches'  dis- 
tance from  it,  and  you  will  find  that  in  a  certain  position 
the  other  wafer  or  ink-spot  will  disappear,  although  it 
is  evidently  still  in  the  field  of  view.    Having  discovered 


OPTICAL  ILLUSIONS. 


37 


this  point  which  differs  for  different  eyes,  you  will  find 
that  if  you  diminish  or  increase  the  distance  of  the 
paper  you  will  once  more  see  the  missing  object.  The 
same  thing  happens  if  you  move  the  eye  from  the  centre 
of  the  wafer.  The  same  experiment  may  be  repeated 
with  the  left  eye  with  a  precisely  similar  result. 

It  has  been  found  by  experiment  that  this  particular 
blind  space  exists  exactly  over  the  base  of  the  optic 
nerve,  at  the  spot  where  it  joins  the  eye.  (Fig.  1.) 
Thus  we  see  that  the  nerve  which  actually  conveys  the 
impression  of  sight  to  the  brain  is  in  itself  incapable  of 
being  excited  by  light.  In  such  cases  as  these  Nature 
seems  to  laugh  at  us,  and  escapes  from  our  grasp  just 
as  we  are  most  confident  in  our  power  of  wresting  her 
secrets  from  her  ;  indeed  we  may  compare  her  to  a  wise 
and  good-natured  mother,  who,  though  always  amiable 
and  willing  to  instruct  those  about  her,  sometimes 
smiles  when  her  children  fancy  they  are  as  learned  as 
she  is. 

If  we  do  not  perceive  the  constant  recurrence  of  the 
phenomenon  just  menti  ned,  it  is  because  when  both 
eyes  are  open  the  object  whose  image  falls  on  the  blind 
spot  in  one  eye  is  seen  by  the  other,  the  insensible 
portions  of  each  eye  being  on  opposite  sides.  Not  only 
this :  the  spot  being  always  situated  on  the  outer  and 
indistinct  portion  of  the  image  reflected  on  the  retina, 
we  do  not  take  notice  of  it ;  for  as  every  one  has  no 
doubt  observed,  it  is  only  the  small  portion  of  the 
object  we  are  looking  at  exactly  opposite  the  centre  of 
the  eye  that  is  perfectly  distinct  and  clear,  all  the  rest 
being  confused  in  its  details,  although  quite  visible. 

Again,  we  may  account  for  our  not  noticing  it  by  the 
fact  of  our  seeiug  clearly  only  those  things  which 
specially  attract  our  attention — a  fact  first  noticed  by 
Mariotte.  We  see  only  what  we  wish  to  see  with  our 
physical  eyes,  as  well  as  those  of  our  mind.    If  our 


38 


THE  WONDERS  OF  OPTICS. 


attention  is  attracted  by  a  particular  portion  of  a  land- 
scape, we  see  only  that,  and  nothing  else.  If  it  is 
fixed  on  some  subject  that  we  are  contemplating  in- 
wardly, we  see  nothing  at  all,  although  our  eyes  may 
not  only  be  wide  open,  but  absolutely  fixed  on  some 
particular  object.  For  instance,  suppose  a  sportsman 
is  out  in  the  fields  preceded  by  his  dogs,  Bran  and 
Ponto.  If  he  follows  the  movements  of  Bran  with 
attention,  he  becomes  the  only  object  animate  or  inani- 
mate, that  depicts  itself  on  his  retina.  Ponto  may 
jump  and  caper  in  vain:  he  is  lost  to  his  master's  eye 
as  much  as  if  he  were  not  there  at  all;  his  mind  is 
entirely  fixed  on  the  beauty  of  Bran's  coat,  on  the  fit 
of  his  collar,  or  fifty  other  things,  and  he  sees  nothing 
else.  But  let  the  sportsman  begin  to  think  of  the 
number  of  birds  he  shot  yesterday,  or  how  he  will  find 
time  to  get  up  to  the  grouse  in  Scotland,  or  of  that 
fine  stag  he  missed  when  he  was  last  amongst  the 
heather,  and  dogs,  cover,  and  landscape  will  fade  from 
his  sight  as  effectually  as  if  he  had  been  struck  with 
blindness.  Let  him,  however,  strike  his  foot  against  a 
stump,  or  let  the  dogs  suddenly  begin  to  point,  and  he 
instantly  receives  back  his  sight,  which  but  a  few 
moments  before  he  had  lost  to  all  intents  and  pur- 
poses. 

The  phenomena  of  ovular  spectra  and  complementary 
colours  experienced  by  every  one  forms  a  curious 
chapter  in  the  history  of  those  illusions  which  take 
their  origin  in  the  eye  itself.  Every  one  has  noticed 
that  after  looking  fixedly  at  a  bright  light  or  a  striking 
colour  for  a  few  moments,  the  eye  preserves  an  impres- 
sion of  the  object  for  a  certain  time.  A  very  light 
window  looked  at  intently  for  several  seconds  will  leave 
the  impression  of  its  cross-bars  on  the  retina  for  several 
minutes,  the  colour  of  the  image  changing  at  every 
movement  of  the  eye.  The  same  effect  may  be  observed 


OPTICAL  ILLUSIONS. 


39 


when  looking  at  the  setting  sun,  or  a  flaring  gas  light 
If  the  light  at  which  we  look  is  coloured,  we  shall  see 
the  complementary  colour  in  the  impression  left  on  the 
retina.  Sir  David  Brewster  was  one  of  the  first  to 
notice  and  experiment  upon  these  very  interesting 
facts. 

If  we  cut  out  any  simple  figure,  a  small  cross  for  in- 
stance, in  scarlet  paper,  place  it  upon  a  white  back- 
ground and  look  at  it  fixelly  for  a  minute  or  two,  we 
shall  find  that  its  tint  will  gradually  become  duller. 
If  we  now  suddenly  look  at  a  piece  of  white  paper, 
we  shall  see  the  cross  depicted  upon  it  in  green,  which 
is  the  complementary  colour  to  red.  It  should  be  ex- 
plained, that  the  complementary  of  any  colour  is  that 
which  is  necessary  to  make  white  light.  Thus,  blue, 
yellow,  and  red  (as  we  shall  find  out  when  wTe  come  to 
speak  of  the  prismatic  spectrum),  mixed  in  certain  pro- 
portions, form  white  light  ;  consequently  the  comple- 
mentary of  orange,  which  is  composed  of  red  and  yel- 
low, will  be  blue  ;  of  green,  which  is  yellow  and  blue, 
red  ;  of  purple,  which  is  blue  and, red,  yellow,  and  vice 
versa.  The  complementary  of  black  is  white,  and  of 
white,  black  as  a  rule  ;  but  if  the  white  object  be  very 
brilliant,  the  black  spectrum  will  speedily  become  col- 
oured. The  impression  left  by  the  setting  sun  is  of  this 
character.  At  first,  while  the  eye  is  open,  the  image 
is  black,  then  brownish  red,  with  a  light  blue  border ; 
but  if  the  eye  be  shut  suddenly,  it  becomes  green,  with 
a  red  border,  the  brilliancy  of  colour  being  apparently 
in  proportion  to  the  strength  of  the  impression.  These 
spectra  may  be  perceived  for  a  long  time,  if  the  eye  is 
gently  rubbed  with  the  finger  now  and  then.  Some 
eyes  are  more  impressionable  in  this  respect  than 
others,  and  Beyle  gives  an  instance  of  an  individual 
who  saw  the  spectrum  of  the  sun  for  years,  whenever 
he  looked  at  a  bright  object.     A  modern  instance  of 


40 


THE  WONDERS  OF  OPTICS. 


this  occurred  lately  to  an  amateur  astronomer  who  was 
looking  at  an  eclipse  of  the  sun.  He  unfortunately 
used  a  glass  that  was  not  sufficiently  smoked,  and  the 
image  of  the  sun's  disc,  with  the  black  space  caused  by 
the  intervening  moon,  remained  on  his  retina  for  months 
after.  This  gentleman's  case  afforded  an  instance  of 
the  necessity  of  attention  in  order  to  see  any  object, 
for  after  the  first  few  days  he  only  became  sensible  of 
his  unfortunate  mishap  when  his  attention  was  called  to 
it  by  some  accidental  circumstance.  These  facts  were 
so  inexplicable  to  Locke,  that  he  consulted  Newton  on 
the  subject,  and  was  surprised  to  learn  that  the  great 
philosopher  himself  had  suffered  for  several  months 
from  a  sun-spectrum  in  the  eye. 

Without  affirming  that  optical  illusions  are  the  cause 
of  all  the  supposed  supernatural  appearances  of  which 
we  have  heard  so  much,  there  is  no  doubt  that  in  many 
instances  the  eye  plays  an  important  part  in  deluding 
the  brain.  The  following  example,  also  cited  by  Beyle, 
will  show  this  clearly.  A  horseman  dressed  in  black, 
and  riding  a  white  hprse,  was  trotting  alorjg  a  portion 
of  the  road,  which  through  a  sudden  break  in  the 
clouds  was  brilliantly  illuminated  by  the  rays  of  the 
sun.  The  black  figure  of  the  man  was  projected 
against  a  white  cloud,  and  the  horse  appeared  doubly 
brilliant  from  being  seen  against  the  dark-coloured  road. 
A  person  who  was  greatly  interested  in  the  arrival  of 
the  horseman  was  watching  them  with  great  attention, 
when  suddenly  the  horse  and  his  rider  disappeared  be* 
hind  a  wood.  An  instant  after  the  observer  was  terri- 
fied at  seeing  a  white  cavalier  on  a  black  horse  project- 
ed on  a  white  cloud  at  which  he  was  accidentally 
looking.  It  may  be  readily  imagined  that  such  an  oc- 
currence, followed  up  by  a  succession  of  unusual  events, 
— such  as  illness,  death,  or  any  other  series  of  misfor- 
tunes,— might  even  in  the  present  day  add  a  chapter  to 
the  history  of  the  marvellous. 


OPTICAL  ILLUSIONS. 


41 


To  the  illusions  to  which,  like  the  preceding,  we  are 
all  subject,  may  be  added  those  resulting  from  some  ab- 
normal conformation,  or  some  disease  of  the  eye,  in 
those  who  labour  under  them.  An  example  of  this  oc- 
curs in  the  case  of  double  or  triple  vision,  many  re- 
markable instances  of  which  are  mentioned  by  Muller, 
the  celebrated  physiologist. 

Although,  as  before  explained,  the  image  of  an  object 
is  depicted  at  the  same  time  on  both  our  eyes,  still  we 
only  see  one  impression,  in  consequence  of  the  two 
images  being  carried  to  the  brain  from  corresponding 
portions  of  the  retina.  If  this  relation  be  disturbed  by 
any  cause,  or  if  the  eyes  are  not  converged  exactly 
upon  the  same  point,  a  double  image  is  the  result. 
The  first  of  these  facts  may  be  proved  by  looking  at 
the  moon,  for  instance,  with  the  left  eye  shut ;  on 
suddenly  opening  it,  two  images  will  be  seen  for  an 
instant.  The  second  is  instantly  proved  by  pushing 
either  of  the  eyes  aside  with  the  finger,  when  looking 
at  any  object. 

It  is  necessary,  however,  to  distinguish  between  these 
effects  and  true  double  vision,  as  well  as  a  certain  defect 
which  exists  in  the  eyes  of  many  people,  consisting  in  the 
apparent  multiplication  of  distant  objects  by  the  same 
eye.  In  these  cases,  there  is  a  superposition  of  images 
upon  the  retina,  each  having  its  proper  bounds.  With 
the  majority  of  individuals  afflicted  in  this  way,  it 
only  happens  when  they  look  at  a  very  distant  object, 
the  moon  or  stars  for  instance.  There  are  many,  how- 
ever, who  suffer  from  it  in  the  case  of  everything  they 
look  at,  whether  far  or  near.  Stephenson,  who  was  affected 
with  it,  made  it  the  subject  of  many  interesting  experi- 
ments. When  he  looked  at  a  clear  mark  on  a  whice 
ground,  and  gradually  walked  away  from  it,  not  only 
did  the  image  become  indistinct,  but  it  seemed  to  u:.fold 
itself  into  several,  independently  of  many  others  much 


42 


THE  WONDERS  OF  OPTICS. 


more  indistinct,  more  especially  two  situated  on  each 
side,  whose  distance  increased  the  farther  he  walked 
away.  As  these  latter  images  became  more  and  more 
separated,  they  also  became  more  confused.  The  image 
seen  by  the  right  eye  was  a  little  higher  than  that 
seen  by  the  left.  Griffin  states,  that  after  having  used 
the  telescope  for  any  length  of  time,  the  eye  that  he 
kept  shut  always  saw  objects  triple  and  double  for 
some  hours  afterwards.  These  phenomena  are  possibly 
connected  in  some  way  with  the  disposition  of  the 
plates  and  fibres  of  which  the  crystalline  lens  of  the 
eye  is  composed. 

Semi-vision,  or  hemiopia  as  it  is  called,  is  much  more 
rare  and  more  difficult  to  explain  than  the  phenomena 
of  double  vision ;  and  consists  in  the  power  of  being 
able  to  see  only  the  right  or  left  half  of  the  object 
looked  at,  the  separation  being  vertical  when  the  eyes 
of  the  observer  are  in  the  same  horizontal  line.  Thus, 
in  looking  at  the  word  Newton,  the  person  so  afflicted 
would  only  see  either  the  letters  New  or  ton  according 
to  which  half  of  the  eyes  were  defective. 

Wollaston  was  afflicted  with  hemiopia  on  two  dif- 
ferent occasions ;  the  first  time  after  violent  exercise, 
during  two  or  three  hours,  when  he  could  see  distinctly 
only  the  left-hand  halves  of  the  objects  he  was  looking 
at.  Both  eyes  were  similarly  affected,  and  the  pheno- 
menon only  lasted  about  a  quarter  of  an  hour.  Twenty 
years  afterwards  he  suffered  again  from  the  same 
accident,  but  on  this  occasion  in  the  contrary  manner; 
that  is  to  say,  he  only  saw  the  right  halves  of  the 
objects  he  was  looking  at — to  use  his  own  words,  he 
could  only  see  the  right  half  of  every  friend  he  met. 
At  certain  distances  from  the  eye,  one  of  two  persons 
would  become  invisible,  and  by  simply  changing 
his  own  position  or  that  of  the  persons  he  was  near, 
he  could  make  one  or  other  of  them,  or  indeed  both, 


OPTICAL  ILLUSIONS. 


43 


disappear  at  will.  It  must  be  acknowledged  that 
similar  tricks  of  Dame  Nature,  due  to  an  unconscious 
insensibility  of  the  eye,  are  most  singular,  and  at  first 
sight  appear  to  hdve  a  supernatural  origin. 

Bartholin  mentions  the  case  of  a  hysterical  woman 
who  was  afflicted  with  hemiopia  horizontally,  and  saw 
all  natural  objects  cut  in  twTo,  the  lower  halves  being 
invisible.  In  this  instance  it  was  only  the  left  eye  that 
was  defective. 

Another  interesting  example  of  optical  illusion  is  the 
luminous  sensation  produced  internally  vihen  the  eye, 
or  the  neighbouring  parts,  are  struck  or  stimulated  by 
friction  or  electricity.  These  appearances  are  experi- 
enced even  by  those  who  have  lost  their  sight.  Muiler 
states  that  a  case  was  submitted  to  a  legal  tribunal  to 
decide  whether  the  luminous  sensations  which  are  per- 
ceptible when  we  rub  our  eyes  are  really  light.  The 
matter  in  dispute  was  wThether  a  man  who  was  attacked 
by  robbers  in  the  dark,  could  see  and  recognise  them  by 
means  of  the  light  produced  in  his  eyes  by  a  violent  blow 
on  the  head  ;  but  he  does  not  tell  us  how  the  question 
was  decided.  With  regard  to  internal  causes,  Hum- 
boldt tells  us  that  a  man  whose  eye  had  been  extirpated, 
was  sensible  of  luminous  appearances  whenever  he  was 
galvanized.  Lincke  states  that  a  man  whose  eye  had 
been  removed  by  a  surgical  operation,  saw  next  day  all 
kinds  of  luminous  phenomena,  which  tormented  him  cru- 
elly with  the  idea  that  after  all  his  eye  had  been  saved. 
When  he  shut  the  perfect  eye,  he  fancied  he  saw  with 
the  missing  eye  circles  of  fire,  persons  dancing,  and  si- 
milar appearances  for  several  days.  These  facts  are 
analogous  to  those  told  of  persons  who  have  had  their 
legs  and  arms  amputated,  but  who,  notwithstanding, 
apparently  feel  pain  in  their  lost  limbs. 


44 


THE  WCWDEiiS  OF  OPTICS. 


CHAPTER  V. 

THE  APPRECIATION  OF  COLOUR. 

Most  people  understand  each  other  sufficiently  to 
agree  in  their  ideas  about  various  colours.  Thus  every 
one  agrees  in  saying  that  poppies  are  red,  that  the  sky 
is  blue,  and  the  leaves  green ;  but  if  any  one  were  to 
assert  that  the  sky  was  red,  that  the  leaves  were  blue, 
and  poppies  green,  who  could  possibly  contradict  him? 

This  statement  may  appear  a  paradox,  and  an  absur- 
dity to  many  of  our  readers,  but  it  is  really  a  problem 
that  has  engaged  the  attention  of  many  of  our  greatest 
philosophers.  Who  can  prove  that  what  I  see  as  yel- 
low may  not  appear  blue  to  you,  or  that  what  you  see 
red  is  not  green  to  me  ?  You  would  possibly  explain 
the  doubt  by  saying  that  because  we  both  agree  in  call- 
ing a  buttercup  yellow,  that  we  see  the  same  colour.  I 
call  a  buttercup  yellow,  because  I  have  learnt  since  my 
childhood  to  give  this  name  to  the  particular  sensation 
I  experience  when  I  look  at  one  of  these  flowers ;  but 
that  is  no  proof  that  the  sensation  I  feel  is  similar  to  that 
felt  by  everybody  else,  and  it  is  not  merely  possible,  but 
probable,  that  our  personal  sensations  of  colour  are  es- 
sentially different,  although  the  arbitrary  words  we  use 
to  designate  them  are  the  same. 

It  may  be  remarked  in  parenthesis,  that  colour  is  not 
an  entity,  but  is  simply  the  effect  of  certain  properties 
of  surface  or  interior  structure  possessed  by  every  sub- 


THE  APPRECIATION  OF  COLOUR. 


45 


stance  with  which  we  are  acquainted.  The  old  saying, 
that  "  all  cats  are  black  in  the  dark/'  is  really  a  pro- 
found philosophical  truth,  which  is  not  only  true  of  cats 
but  of  the  reddest  rose  that  ever  grew  in  a  garden,  the 
bluest  violet  that  ever  was  plucked,  the  prettiest  girl 
that  ever  was  kissed  under  the  mistletoe.  It  is  a  sad 
thing  to  think  of,  that  when  we  put  the  candle  out,  and 
step  into  bed,  we  become  blacker  than  the  blackest  ne- 
gro that  was  ever  emancipated.  But  without  light 
there  can  be  no  colour,  for  there  is  no  material,  so  to 
speak,  from  which  to  manufacture  it.  White  light,  as 
we  have  said  before,  is  made  up  of  .red,  blue,  and  yel- 
low, and  it  is  by  tfoe  absorption  of  one  or  all  of  these 
that  all  tints  are  formed.  The  surface  of  a  poppy  leaf 
has  the  power  of  absorbing  all  the  blue  and  a  little  of 
the  yellow,  reflecting  the  whole  of  the  red  and  the  re- 
mainder of  the  yellow,  the  mixture  of  the  two  forming 
scarlet.  The  surface  of  a  marigold  acts  differently;  all 
the  blue  is  absorbed,  as  in  the  case  of  the  poppy,  and  a 
good  deal  of  the  red  with  it,  leaving  just  a  little  to 
brighten  up  the  yellow  which  is  reflected  with  it.  Some 
substances,  white  marble  for  instance,  have  no  power 
of  splitting  the  light  into  colours,  absorbing  some  and 
reflecting  others,  but  reflect  the  whole  of  it  in  its  inte- 
grity. Others  again,  like  black  velvet,  absorb  nearly 
the  whole,  just  reflecting  sufficient  to  enable  us  to  see 
its  surface. 

We  began  this  chapter  by  speculating  on  the  proba- 
bility of  our  seeing  different  colours  to  our  neighbours, 
and  we  shall  now  proceed  to  show  that  our  speculations 
in  that  direction  are  not  so  absurd  as  they  appear  to  be 
at  first  sight. 

The  phenomenon  of  colour  blindness,  or  the  insensi- 
bility of  the  eye  to  certain  colours,  has  been  for  many 
years  past  a  puzzle  both  to  the  physiologist  and  the 
philosopher.    Perhaps  the  most  remarkable  case  of  the 


i 


46 


THE  WONDERS  OF  OPTICS. 


sort  is  that  mentioned  first  by  Huddart,  and  quoted  by 
Sir  David  Brewster,  of  a  shoemaker  named  Harris, 
living  at  Maryport,  in  Cumberland,  who  was  utterly 
incapable  of  distinguishing  any  colour  at  all,  and  saw 
everything  white,  grey  or  black.  The  first  time  that 
Harris  noticed  this  defect,  was  when  he  was  about  four 
years  old ;  having  found  the  stocking  of  a  playmate  in 
the  street,  he  returned  it  to  him  at  his  cottage,  and 
noticed  that  every  one  said  it  was  a  red  stocking,  but  he 
could  not  understand  why  they  should  call  this  par- 
ticular stocking  red,  as  it  seemed  to  him  to  be  like  every 
other.  This  circumstance  remained  in  his  mind,  and  a 
few  more  similar  observations  confirmed  his  suspicions 
that  he  had  some  defect  of  sight  that  prevented  hirn 
from  seeing  as  others  did.  He  also  observed  that  other 
children  pretended  to  distinguish  cherries  from  their 
leaves  by  what  they  called  their  colour,  whilst  he  could 
see  no  difference  between  them,  except  those  of  shape 
and  size.  He  also  noticed  that  by  means  of  the  differ- 
ence of  colour,  others  could  distinguish  cherries  on  a  tree 
at  a  much  greater  distance  than  he  could ;  whilst  he,  on 
the  contrary,  could  see  other  things  at  greater  distances 
than  his  companions.  Harris  had  two  brothers,  whose 
eyes  were  similarly  defective ;  one  of  these,  that  Hud- 
dart examined,  mistook  green  for  yellow  constantly,  and 
orange  for  light  green. 

In  the  Philosophical  Transactions  Scott  describes  a 
similar  defect  in  his  own  powers  of  vision.  He  states 
that  he  was  unable  to  distinguish  green,  and  that  the 
colours  known  as  crimson  and  pale  'blue  presented  no 
difference  of  hue.  He  further  confesses  his  inability  to 
see  any  difference  between  bright  green  and  bright  red, 
although  he  could  distinguish  between  red  and  yellow, 
dark  blue,  and  almost  every  shade  of  blue,  except  sky- 
blue.  He  goes  on  to  relate  how  he  married  his  daugh- 
ter to  a  worthy  young  man  of  his  acquaintance,  and  that 


THE  APPRECIATION  OF  COLOUR. 


47 


the  day  before  the  wedding  the  bridegroom  came  to  his 
house  in  a  full  suit  of  black,  as  he  thought.  He  was 
greatly  displeased  to  see  him  appear  in  mourning  on 
such  an  occasion,  and  took  an  opportunity  to  remonstrate 
with  him  on  the  subject.  But  what  was  his  surprise  to 
hear  his  daughter  exclaim,  in  loud  tones  of  counter  re- 
monstrance, that  she  had  rarely  seen  her  lover  in  a  coat 
of  such  a  pretty  colour,  and  that  her  father's  eyes  must 
deceive  him  on  this  as  on  many  other  occasions.  Scott's 
father,  his  maternal  uncle,  one  of  his  sisters,  and  two  of 
his  sons  had  the  same  defect  of  sight.  Dr.  Mitchell 
mentions  the  case  of  a  naval  officer  who  for  his  ordinary 
uniform  chose  a  blue  coat  and  waistcoat  and  red  trousers, 
fully  believing  that  they  were  all  of  the  same  colour.  A 
tailor  of  Plymouth,  also  mentioned  by  Dr.  Mitchell, 
mended  a  black  silk  waistcoat  with  a  piece  of  crimson, 
and  another  put  a  red  cloth  collar  to  a  blue  coat.  Se- 
veral celebrated  men  have  suffered  from  colour-blindness. 
Amongst  them  may  be  mentioned  Dugald  Stewart,  the 
great  philosophical  writer  ;  John  Dalton,  the  originator 
of  the  atomic  theory;  and  Troughton,  the  philosophical 
instrument  maker.  Dugald  Stewart  first  discovered  the 
defect  on  hearing  a  member  of  his  family  admire  the 
contrast  of  colour  between  the  leaves  and  fruit  of  a  Si- 
berian crab-tree,  while  he  could  see  no  difference  between 
them,  except  in  point  of  form  and  size.  John  Dalton 
could  not  distinguish  blue  from  crimson,  and  he  could 
only  see  two  colours,  blue  and  yellow,  in  the  prismatic 
spectrum.  Troughton  could  see  no  difference  between 
dark  crimson,  bright  orange,  and  yellow — in  fact,  he 
could  only  distinguish  blue  from  yellow. 

In  an  article  on  this  subject,  published  in  the  Maga- 
sin  Pittoresque  for  1846,  a  Swiss  physician  gives  some 
interesting  examples,  which  are  worth  repeating.  In 
the  solar  spectrum  obtained  by  passing  a  ray  of  light 
through  a  triangular  prism,  and  which  is  composed  of 


48 


THE  WONDERS  OF  OPTICS. 


the  following  colours, — red,  orange,  yellow,  green,  blue, 
indigo,  and  violet, — Dalton  could  only  see  yellow,  blue, 
and  violet.  Rose-colour  by  day  appeared  to  him  a  pale 
blue,  but  at  night  it  seemed  to  take  an  orange  hue.  By 
day  crimson  seemed  to  be  dirty  blue,  and  red  cloth  dark 
blue.  Dr.  Whewell  having  asked  him  one  day  to  describe 
the  colour  of  the  doctor's  scarlet  gown,  Daiton  pointed 
to  the  trees  around  them,  and  declared  he  could  distin- 
guish no  difference  in  their  colour  ;  and  one  day  having 
dropped  a  stick  of  red  sealing-wax  in  the  grass,  he  had 
the  greatest  difficulty  in  finding  it  again.  Since  Dal- 
ton's  time  over  five  hundred  distinctly  marked  instances 
of  this  imperfection  have  been  noticed,  and  Professor 
Prevost,  of  Geneva,  has  named  it  Daltonism,  an  ex- 
tremely unphilosophical  piece  of  pathological  nomen- 
clature, which  has  unfortunately  received  the  sanction 
of  too  many  great  physiologists  to  be  abolished.  Blind- 
ness might  just  as  well  be  called  Homerism  or  Mil- 
tonisrn. 

Colour-blindness  is  much  more  frequent  than  is  gene- 
rally supposed,  for  those  who  are  afflicted  with  it  are 
mostly  ignorant  of  the  defect,  and  frequently  practise 
trades  or  professions  in  which  perfect  sensibility  to  the 
different  hues  of  colour  is  quite  indispensable.  An  in- 
stance of  this  occurred  some  time  since  in  the  case  of 
an  engine-driver,  who  allowed  his  engine  to  run  into  a 
luggage  train,  through  not  noticing  the  red  danger  sig- 
nal. At  his  examination  it  was  proved  that  he  was 
colour-blind,  and  could  not  distinguish  red  from  green. 
Partial  colour-blindness  is,  no  doubt,  the  cause  of  the 
frequent  disputes  that  we  hear  about  the  tints  of  cer- 
tain objects  ;  to  say  nothing  of  the  glaring  instances  of 
bad  taste  in  the  arrangement  of  colour  that  are  now-a- 
days  so  common.  Out  of  forty  boys  at  a  school  at  Ber- 
lin who  were  examined  by  Leebech,  he  found  five  who 
were  quite  confused  in  their  notions  of  colour,  and 


THE  APPRECIATION  OF  COLOUR. 


49 


could  not  distinguish  between  ordinary  shades  of  the 
same  hue.  This  affliction  is  in  many  cases  hereditary, 
descending  from  father  to  son.  It  is  singular  that  in- 
stances of  colour-blindness  are  much  more  common 
amongst  men  than  amongst  women,  for  out  of  over  five 
hundred  cases  there  were  only  four  in  which  females 
were  the  sufferers.  It  seems  also  that  persons  with 
grey  eyes  are  more  frequently  colour-blind  than  those 
whose  eyes  are  blue  or  brown.  To  the  list  of  great 
men  who  were  colour-blind,  we  must  not  forget  to  add 
the  celebrated  Italian  historian,  Sismondi. 

Physiologists  consider  that  there  are  two  kinds  of 
colour-blindaess, — one  where  only  two  colours  are  seen, 
the  other  where  more  than  two  are  perceptible.  Dau- 
beny  Turberville,  an  oculist  of  Salisbury,  mentions  a 
case  of  the  former,  in  which  a  young  girl,  like  the  Mary- 
port  shoemaker  mentioned  by  Brewster,  could  only  dis- 
tinguish between  black  and  white,  everything  between 
the  two  being  of  different  shades  of  grey.  This  girl,  sin- 
gularly enough,  could  see  to  read  in  twilight  a  quarter 
af  an  hour  after  her  companions.  This  sharpness  of  sight 
appears  to  be  not  at  all  uncommon  amongst  those  who 
are  colour-blind.  Spurzheim  mentions  the  ease  of  a 
whole  family  who  were  afflicted  in  the  same  way  as 
Turberville's  patient.  All  the  male  members  of  Trough- 
ton's  family  were  equally  incapable  of  distinguishing  any 
colours  but  blue  and  yellow. 

The  cases  of  colour-blindness  where  more  than  two 
colours  are  dis'inguishable,  are  much  more  common. 
Goethe,  the  great  German  poet,  who  dabbled  a  great 
deal  in  optics,  knew  two  young  men  who,  although  they 
possessed  powerful  sight,  and  could  distinguish  between 
white,  black,  grey,  yellow,  and  orange,  were  at  a  loss 
when  the  shades  between  dark  red  and  rose  colour  were 
in  question.  A  piece  of  dried  carmine  appeared  bright 
red  to  them,  and  a  faint  carmine  hue  on  a  white  shell,, 


50 


THE  WOJnDERS  OF  OPTICS. 


and  a  rose-leaf,  light  blue  ;  the  leaves  of  trees  and  grass 
appeared  yellow,  and  they  confounded  rose-colour,  blue, 
and  violet  together.  Goethe  supposed  them  to  be  inca- 
pable of  perceiving  blue  and  its  several  hues,  and  called 
their  defect  by  a  high  sounding  Greek  name,  akyano- 
blepsy,  or  blue-blindness.  Peclet  mentions  two  other 
persons,  also  brothers,  who  likewise  were  incapable  of 
distinguishing  between  blue,  violet,  and  rose-colour. 
Like  Professor  Whewell,  they  confounded  the  dull  scar- 
let of  the  trousers  of  the  French  infantry  with  the 
leaves  of  the  trees.  Yellow  appeared  to  them  more  bril- 
liant than  any  other  colour.  Doctor  Sommer  and  his 
brother  could  not  distinguish  between  red  and  its  deri- 
vatives and  other  colours;  they  could  only  distinguish 
between  yellow,  blue,  white,  and  black.  Doctor  Ni- 
choll  mentions  a  child  that  could  only  see  red,  yellow, 
and  blue,  in  the  spectrum.  It  could  distinguish  green, 
but  called  it  brown  when  it  was  dark,  and  pink  when  it 
was  pale.  The  same  physiologist  knew  a  man  who 
called  red  green,  and  brown  dark  green.  A  young  lady 
who  was  an  amateur  artist,  could  not  perceive  a  piece  of 
scarlet  cloth  hanging  on  a  hedge  that  was  close  to  her, 
although  others  could  see  it  plainly  half  a  mile  off.  One 
day  she  gathered,  as  a  great  curiosity,  a  lichen  which 
she  supposed  to  be  of  a  bright  scarlet  hue,  but  which 
wras  in  reality  of  a  beautiful  green.  Another  time  she 
could  see  no  difference  between  carmine  and  prussian 
blue.  A  gardener  living  at  Clydesdale,  who  began  life 
as  a  weaver,  was  compelled  to  give  up  his  first  trade  be- 
cause in  daylight  he  confounded  all  light  colours ;  yel- 
low and  its  varieties  he  could  distinguish  perfectly,  but 
he  was  incapable  of  seeing  any  difference  between  red, 
blue,  pink,  brown,  and  white.  Another  man,  who  was  a 
silk-weaver,  had  to  change  his  trade,  because  he  could 
not  distinguish  between  red,  pink,  and  sky  blue.  A 
Genevese  artist  whom  circumstances  compelled  to  paint 


THE  APPRECIATION  OF  COLOUR. 


51 


a  portrait  by  candle-light,  used  yellow  for  pink  in  laying 
on  his  flesh  tints,  with  a  pleasing  result  that  may  be 
readily  imagined.  In  fact,  the  instances  of  colour- 
blindness mentioned  by  physiological  writers  are  almost 
innumerable,  and  I  should  only  weary  my  readers  if  I 
related  all  the  authentic  cases  of  this  singular  affliction, 
One  instance,  however,  which  was  very  carefully  ob* 
served  by  Wartmann,  a  distinguished  German  oculist, 
merits  our  attention.  The  afflicted  person,  whom  Wart- 
mann speaks  of  as  D.,  was  thirty-three  years  old.  Those 
of  his  brothers  and  sisters  whose  hair  was  fair  suffered 
from  the  same  infirmity,  but  those  whose  hair  was  dark 
were  exemptfrom  it.  Likeso  many  others  who  are  colour- 
blind, he  could  riot  distinguish  between  cherries  and 
their  leaves,  and  confounded  a  sea-green  piece  of  paper 
with  a  scarlet  ribbon  placed  near  it.  A  rose  of  the 
ordinary  hue  appeared  greenish-blue.  Being  anxious  to 
see  if  reflected,  refracted,  and  polarized  light  exercised 
fi  different  action  on  his  retina,  Wartmann  tried  him  first 
with  the  prismatic  spectrum,  but  he  could  only  distin- 
guish four  colours, — blue,  green,  yellow,  and  red.  He 
could  distinguish  perfectly  the  peculiar  black  lines  seen 
crossing  the  spectrum  in  certain  places,  and  known  by 
the  name  of  Fraunhofer's  lines.  He  then  placed  in  his 
hands  thirty-seven  pieces  of  differently  coloured  glass, 
but  he  could  only  distinguish  four  varieties.  The  colours 
produced  by  polarized  light  seemed  to  give  the  patient 
quite  as  much  trouble  as  those  produced  in  the  ordinary 
way.  Chocolate  brown  appeared  reddish  brown ;  purple, 
dark  blue  ;  and  violet,  a  dirty  blue.  When  colours  were 
illuminated  by  sunlight,  they  seemed  to  him  to  be  redder 
than  usual,  even  green  and  blue  appearing  red. 

In  considering  cases  of  colour-blindness,  it  is  very 
difficult  not  to  be  misled  into  using  wrong  terms,  as  ap- 
plied to  colour,  for  we  have  no  possible  means  of  know- 
ing what  colour  it  is  that  is  really  seen  by  the  patient. 


52 


THE  WONDERS  OF  OPTICS. 


Thus,  for  instance,  Dr.  Whe well  could  not  distinguish 
between  red  and  green.  But  what  colour  did  he  really 
see  ?  Did  he  see  the  leaves  and  cherries  both  red  or 
both  green,  or  was  it  some  colour  between  the  two  that 
was  impressed  upon  his  retina  ?  Again,  great  care 
must  be  exercised  in  placing  implicit  reliance  on  the 
statement  of  persons  who  are  colour-blind,  for  we  must 
recollect  that  their  only  means  of  conveying  the  results 
of  what  they  experience  is  by  the  use  of  an  organ  that 
is  confessedly  defective,  and  which  is  quite  likely  to 
deceive  them,  and  us  too,  without  their  being  parties  to 
the  deception. 

The  cause  of  colour-blindness  is  completely  unknown  ; 
philosophers  and  physiologists  are  still  in  the  realms  of 
hypothesis  concerning  this  peculiar  optical  defect.  As 
yet,  the  most  careful  observation  has  failed  to  detect 
any  difference  between  the  eyes  of  those  who  are  colour- 
blind, and  the  eyes  of  ordinary  persons,  that  could  in 
any  way  account  for  this  singular  affection  of  the  sense 
of  sight. 


ILLUSIONS  CAUSED  BY  LIGHT  ITSELF. 


53 


CHAPTER  VI 

ILLUSIONS  CAUSED  BY  LIGHT  ITSELF. 

When  playing  about  the  Christmas  fire,  children 
frequently  amuse  themselves  by  whirling  round  and 
round  apiece  of  wood,  one  end  of  which  they  have 
previously  lighted  and  blown  out.  In  proportion  as 
the  movement  becomes  more  rapid,  the  path  of  the  red- 
hot  end  becomes  more  and  more  connected,  until  at  last 
a  burning  ring  is  formed,  in  every  part  of  which  the 
shining  charcoal  appears  to  be  at  the  same  time.  The 
only  way  of  accounting  for  this  illusion  is  by  supposing 
that  the  image  formed  by  the  burning  stick  upon  the 
retina  remains  there  for  an  appreciable  period,  the  im- 
pression made  by  it  at  one  part  of  its  journey  remain- 
ing until  it  returns  to  its  former  position.  The  power 
possessed  by  the  retina  of.  retaining  impressions 
explains  a  large  number  of  illusions  of  the  same  kind. 
The  chord  of  a  musical  instrument,  for  instance,  when 
struck,  appears  to  occupy  a  longer  space  during  the 
time  it  vibrates,  than  when  it  is  at  rest.  A  rapidly  re- 
volving wheel  appears  almost  solid  on  account  of  the 
combined  images  of  the  spokes  seeming  to  unite  into 
one  homogeneous  mass. 

The  persistence  of  luminous  impressions  upon  the 
retina  has  given  rise  to  the  invention  of  a  number  of 
well-known  optical  toys,  amongst  which  may  be 
mentioned  the  phenakistiscope,  the  thaumatrope,  the 
phantascope,  and  many  others. 


54 


THE  WONDERS  OF  OPTICS. 


The  phenakistiscope  may  be  described  (figs.  3  and  4) 
as  consisting  of  an  iron  pin  a  b  turning  easily  on  its 
«vxis,  and  passing  through  two  holes  in  a  brass  rod  t  g, 
bent  twice  at  right  angles.  Attached 
r    *         to  one  end  of  'the  pin  is  a  disc  of  card- 
l^^^Jl£a     board,  divided  into  several  equal  sec- 
\j         tors,  and  pierced  near  its  circumference 
with  as  many  similar  sized  rectangular 
holes  (fig.  4  )    In  each  sector  the  same 
scene  is  represented,  with  this  differ- 
ence, that  the  movements  of  the  objects 
e  are  so  arranged  as  to  be  progressive 

from  one  extreme  to  the  other.  The 
disc  being  fastened  to  the  pin  a  b  (fig.  3) 
by  the  screw  v,  with  the  figures  facing 
outwards  towards  a,  the  whole  ap- 
paratus is  held  before  a  looking-glass 
by  the  handle  m.  If  the  disc  be  now 
FIG.3._The  Phena-  rotated  by  the  button  b,  and  the  eye 
kistiscope.  placed  opposite  one  of  the  square  holes 
in  the  card,  the  figures  on  the  disc  will  appear  to  move 
more  or  less  quickly  according  to  the  rate  at  which  it  is 
rotated.  The  three  bricklayers  in  fig.  4  will  be  seen 
to  pass  their  bricks  from  one  to  the  other  with  perfect 
regularity  if  the  drawing  has  been  made  carefully. 
Numberless  other  designs  may  be  made  for  this  little 
instrument,  such  as  a  windmill  in  full  sail,  a  man  work- 
ing a  pump,  a  conjurer  swallowing  knives — in  fact,  any 
scene  with  objects  in  motion  may  be  drawn,  and  will 
cause  infinite  amusement  for  the  long  winter  evenings. 

The  time  during  which  the  impression  of  any  object 
remains  upon  the  retina  appears  to  be  in  direct  propor- 
tion to  its  brilliancy.  For  a  burning  coal  it  is  stated 
to  be  about  the  tenth  of  a  second;  consequently,  if  the 
stick  mentioned  at  the  beginning  of  the  chapter  is 
rotated  ten  times  in  a  second,  a  continuous  luminous 


ILLUSIONS  CAUSED  BY  LIGHT  ITSELF. 


55 


ring  will  appear  to  be  formed.  That  the  time  necessary 
for  producing  a  distinct  impression  varies  with  the 
brilliancy  of  the  object,  may  be  readily  guessed  from 


FlG.  4.— Disc  of  th^  Phenak*stiscope. 


the  fact  that  an  electric  spark  is  perfectly  visible, 
although  its  duration  can  hardly  be  measured,  while  a 
cannon-ball  in  flight  is  only  perceptible  to  the  practised 
eye  of  the  artilleryman,  owing  to  its  reflecting  only  a 
small  quantity  of  very  diffused  light. 

The  second  instrument,  the  thaumatrope,  is  construct- 
ed on  the  same  principle.  It  consists  of  a  certain 
number  of  circular  discs  of  card  three  or  four  inches 
across,  which  are  capable  of  being  turne  1  on  their  axes 


56 


THE  WONDERS  OF  OPTICS. 


with  great  rapidity  by  means  of  the  finger  and  thumb  and 
a  couple  of  silk  threads  fixed  at  opposite  sides  of  their 
circumference.  On  each  of  these  discs  a  design  is 
painted,  one  half  appearing  on  one  side,  and  the  other 
half  on  the  other,  in  such  a  manner  that  the  two  parts 
form  a  single  picture.  You  may  have,  for  instance, 
Harlequin  on  one  side  and  Columbine  on  the  other,  but 
on  turning  the-  card  you  will  see  them  together.  The 
body  of  a  Turk  may  be  drawn  on  one  side  and  his  head 
on  the  other,  and,  by  rotating  the  card,  the  head  sud- 
denly finds  a  pair  of  shoulders  to  fit  it.  A  sentence 
may  be  divided  in  the  same  way,  or  the  words,  or  even 
the  letters,  may  be  divided  between  the  opposite  sides 
of  the  card:  in  fact,  like  the  phenakistiscope,  the  de- 
signs applicable  to  this  little  instrument  are  endless. 

The  third  of  these  instruments,  the  phantascope,  is 
constructed  in  accordance  with  the  peculiar  power  pos- 
sessed by  the  eyes  of  adapting  themselves  to  the  dis- 
tance of  the  objects  they  are  looking  at.  Everybody 
must  have  noticed  that  in  order  to  see  objects  plainly 
that  are  placed  at  different  distances  wTe  insensibly  alter 
the  position  and  focus  of  the  eyes,  and  that,  conse- 
quently, objects  even  in  the  same  plane  as  those  we  are 
looking  at  are  not  perceived  by  us  until  something  calls 
our  attention  to  them,  and  causes  us  to  alter  the  posi- 
tion and  focus  of  our  eyes  and  fix  our  gaze  on  them. 
For  instance,  in  looking  at  a  canary  in  a  cage,  we  have 
but  a  confused  idea  of  the  wires,  which  we  will  suppose 
to  be  midway  between  the  bird  and  the  observer.  But 
if  anything  attracts  our  attention  to  the  wires  we  lose 
sight  of  the  bird,  or  at  any  rate  Fee  it  only  as  a  con- 
fused mass.  If  this  experiment  is  made  with  care,  it 
will  be  perceived  that  the  object  seen  confusedly  is 
always  double,— a  fact  that  may  be  verified  by  inter- 
posing the  finger  between  the  eyes  and  any  object. 
When  we  look  at  the  finger,  the  distant  object  will  se<m 


ILLUSIONS  CAUSED  BY  LIGHT  ITSELF. 


57 


to  be  doubled ;  if  we  look  at  the  object,  it  is  the  finger 
that  undergoes  duplication. 

We  know  by  experience  that  when  we  look  at  an  ob- 
ject and  press  one  of  the  eyeballs  slightly  with  the 
finger,  the  image  of  it  becomes  doubled.  The  explana- 
tion of  this  phenomenon  is  not  very  easy,  but  it  is  gen- 
erally supposed  that  in  the  case  of  ordinary  vision  the 
two  eyes  produce  the  sensation  of  a  single  image  in 
consequence  of  the  two  impressions  being  formed  at 
corresponding  parts  of  each  retina,  and  that  habit 
causes  us  to  see  only  a  single  object  in  such  a  case. 
But  when  the  eyes  are  so  disposed  as  to  be  capable 
of  seeing  distant  objects  distinctly,  the  two  images 
formed  by  a  near  object  are  no  longer  found  in  the  cor- 
responding portions  of  each  retina,  and  so  produce  the 
sensation  of  double  vision.  The  same  thing  happens 
when  either  of  the  eyes  is  momentarily  displaced. 

These  phenomena  have  given  rise  to  the  construction 
of  a  very  simple  instrument,  the  phantascope,  with 
which  many  interesting  experiments  may  be  performed, 
and  which  was  invented  some  years  since  by  Dr.  Lake, 
an  eminent  physician  of  New  York. 

In  the  middle  of  one  of  the  edges  of  a  thin  piece  of 
wood,  say  six  inches  or  a  foot  in  length,  which  serves 
as  a  base  for  the  instrument,  is  fixed  a  rod  fourteen  or 
sixteen  inches  long,  upon  which  slide  a  couple  of  fer- 
ules capable  of  being  fixed  at  any  height  by  means  of 
thumb-screws.  Each  of  these  ferules  holds  a  piece  of 
cardboard  five  or  six  inches  long,  and  of  any  conve- 
nient breadth,  in  a  horizontal  position.  The  upper  card  is 
pierced  in  a  longitudinal  direction  with  a  slit  rather  less 
than  a  quarter  of  an  inch  broad,  and  about  three  inches 
long ;  that  is  to  say,  a  little  wider  than  the  distance  be- 
tween the  centres  of  two  eyes.  The  second  card  has  a 
similar  slit  of  the  same  length,  and  corresponding  ver- 
tically with  the  one  above  it ;  the  width,  however,  in 


58 


THE  WONDERS  OF  OPTICS. 


this  instance  being  only  about  the  eighth  of  an  inch.  In 
addition,  the  lower  card  should  be  marked  with  a  fine 
line  drawn  across  the  centre,  which  we  shall  call  the  in- 
dex. 

Things  being  thus  arranged,  if  we  place  two  similar 
objects — two  A's,  for  instance — upon  the  wooden  stage 
of  the  instrument,  about  three  inches  apart,  and  look 
at  them  through  the  two  slits,  we  shall  see  them  as  un- 
der ordinary  circumstances ;  but  on  fixing  our  eyes  in- 
tently on  the  index  of  the  lower  card,  and  gradually 
raising  it,  we  shall  see  the  two  A's  become  double,  the 
two  images  of  each  letter  separating  themselves  more 
and  more  the  nearer  the  lower  card  approaches  the  up- 
per one,  until  the  last  two  of  the  images  will  coalesce, 
and  appear  to  be  placed  on  the  lower  cardboard,  the 
other  two  remaining  in  their  proper  place.  The  eyes 
must  be  kept  firmly  fixed  upon  the  index,  otherwise  the 
illusion  disappears  immediately,  and  two  A's  only  are 
seen  in  their  true  position  on  the  base  of  the  instrument. 
This  is  an  instance  of  the  production  of  an  image  in  a 
place  where  it  certainly  does  not  exist.  This  illusion 
is  seen  best  when  the  upper  screen  is  about  ten  inches 
from  tha  object,  the  lower  screen  being  just  half-way 
between ;  but,  as  in  most  of  these  cases,  the  distances 
will  differ  according  to  the  focus  of  the  observer's  eyes. 
The  proper  distances  once  being  found,  the  experiment 
may  be  varied  in  a  hundred  different  ways.  For  ex- 
ample, instead  of  two  letters  and  a  line  we  may  have 
two  flowers  on  the  stage,  and  the  figure  of  a  flower-pot 
on  the  intermediate  screen.  If  the  two  flowers  are 
painted"  different  colours,  they  will  unite  and  form  a 
mixed  tint.  Thus  a  red  and  yellow  flower  will  give  an 
orange  image,  a  blue  and  yellow  a  green  image,  and  so 
on.  A  perpendicular  stroke  and  a  horizontal  one  will 
give  a  cross.  A  few  experiments  with  this  little  instru- 
ment will  throw  a  light  upon  many  of  the  obscurer 


ILLUSIONS  CAUSED  BY  LIGHT  ITSELF. 


59 


points  that  exist  amongst  the  phenomena  of  vision,  and 
will  show  conclusively  that  the  two  eyes  rarely  see  in 
the  same  manner,  and  that  it  is  sometimes  one,  and 
sometimes  the  other,  that  sees  most  distinctly.  A 
couple  of  pieces  of  cardboard,  pierced  with  suitable 
slits  and  held  in  the  hand  may  be  substituted  for  the 
apparatus  above  described,  but  of  course  they  will  be 
much  more  difficult  to  use,  and  will  give  less  satisfac- 
tory results. 


60 


THE  WONDERS  OF  OPTICS. 


CHAPTER  VII. 

THE  INFLUENCE  OF  THE  IMAGINATION. 

The  above  facts  show  plainly  that  optical  illusions 
find  their  source  in  the  very  mechanism  of  the  organs 
of  sight,  and  that  without  going  farther  than  the  eye 
itself  we  may  discover  numberless  examples  of  these 
phenomena.  We  shall  presently  bring  before  our  read- 
ers the  innumerable  means  devised  by  art  for  deceiving 
the  sense  of  sight  and  impressing  us  with  sensations  that 
are  purely  imaginary.  But  before  describing  these  nu- 
merous pieces  of  apparatus  we  must  still  remain  for  a 
short  time  within  the  domain  of  man's  faculties,  and  de- 
scribe some  of  the  illusions  that  we  are  subjected  to  by 
those  powers  of  the  imagination  that  are  supposed  to 
hold  in  check  the  five  senses  of  the  body.  Our  imagi- 
nation, however,  plays  us  as  many  tricks  as  our  eyes, 
and,  like  them,  is  alternately  false  and  true.  Touch, 
taste,  smell,  hearing,  and  sight,  are  all  supposed  to  be 
under  its  powerful  influence  for  good  or  evil;  but  they 
are  all  deceived  by  it  in  turn,  more  especially  the  sense 
of  sight,  which  we  generally  boast  of  as  being  the  most 
trustworthy  of  them  all.  Were  we  to  describe  all  the 
labyrinths  into  wThich  our  imagination  is  continually 
leading  us,  we  might  easily  extend  this  little  volume  to 
one  of  treble  the  size.  But  our  purpose  is  not  so  much 
to  write  a  history  of  all  the  hallucinations  to  which  the 
imagination  is  subject,  but  to  cull  from  those  already 


THE  INFLUENCE  OF  THE  IMAGINATION.  61 


existing  the  most  interesting  instances  in  which  this 
great  faculty  is  alternately  the  victim  and  the  tyrant  of 
the  sense  of  sight. 

Amongst  many  works  on  this  subject  we  may  cite  that 
of  Briere  de  Boismont  on  "  Hallucinations,  Apparitions, 
Visions,  &c,"  from  which  we  shall  draw  largely  in  the 
following  pages.  The  examples  wfe  shall  give  will  be 
those  only  in  which  the  victims  of  the  hallucination  were 
in  the  full  enjoyment  of  their  mental  faculties,  and  could 
healthily  analyze  the  sensations  and  impressions  to  which 
they  were  subjected. 

One  of  the  first  of  these  bears  upon  those  diseases  of 
the  eye  to  which  allusion  was  made  in  Chapter  IV. 
Towards  the  end  of  1833,  a  poor  washerwoman  who  was 
tormented  grievously  wTith  rheumatic  pains  gave  up  her 
business,  and  took  to  sewing  for  her  livelihood.  Being 
but  little  accustomed  to  this  kind  of  wTork,  she  was  com- 
pelled to  sit  over  her  needle  late  at  night  in  order  to  save 
herself  from  starving.  The  unwonted  strain  upon  the 
eyes  soon  brought  on  ophthalmia,  wrhich  speedily  became 
chronic.  Nevertheless,  she  continued  her  work,  and 
fell  a  prey  to  diplojria,  or  double  sight  in  each  eye. 
Instead  of  a  single  needle  and  thread,  she  saw  four  con- 
tinually at  work,  everything  else  about  her  being  simi- 
larly multiplied.  At  first  she  took  no  notice  of  the  sin- 
gular illusion,  but  at  last  both  imagination  and  sight 
joined  arms  against  the  judgment,  and  the  poor  crea- 
ture imagined  that  Providence  had  taken  pity  on  her 
forlorn  condition,  and  had  worked  a  miracle  in  her 
favour  by  bestowing  on  her  four  pair  of  hands  in 
order  that  she  might  do  four  times  her  usual  amount  of 
work. 

The  following  is  another  instance  of  the  passage  of 
illusion  into  hallucination.  A  man  fifty-two  years  old, 
of  a  plethoric  constitution,  after  having  suffered  from  a 
defect  in  his  visual  functions  that  caused  him  to  see  ob- 


62 


THE  WONDERS  OF  OPTICS. 


jects  sometimes  double,  and  hi  others  upside  down,  sud- 
denly showed  signs  of  cerebral  congestion,  and  threat- 
ened apoplexy.  By  proper  treatment,  however,  he  was 
saved  for  a  time  from  the  latter  catastrophe,  but  he  be- 
came permanently  afflicted  with  strabismus,  or  squint- 
ing, and  he  suffered  from  a  singular  hallucination.  His 
eyelids  would  contract,  and  his  eyeballs  would  roll  from 
side  to  side  at  more  or  less  distant  intervals.  On  these 
occasions  he  imagined  he  saw  the  figures  of  different 
persons  that  he  knew  moving  about,  and  would  even 
follow  them  outside  his  door  into  the  other  rooms  of  the 
house.  He  was  perfectly  aware  that  these  appearances 
were  merely  the  effect  of  the  imagination,  but  this  did 
not  in  any  way  detract  from  their  appearance  of  reality. 
The  man  afterwards  died  from  an  attack  of  apoplexy. 

The  following  examples  are  also  cases  of  singular 
optical  deception,  some  of  them  being  so  extraordinary 
as  to  trench  upon  the  supernatural,  and  in  the  days  of 
ignorance  would  have  given  those  who  were  their  vic- 
tims the  character  of  unearthly  personages. 

A  certain  English  painter,  who  in  some  sort  inheri- 
ted the  palette  of  Sir  Joshua  Reynolds,  and  believed 
himself  superior  in  many  respects  to  the  great  master, 
used  to  boast  that  in  one  year  he  painted  over  three 
hundred  portraits,  large  and  small.  This  fact  seemed 
to  Wigan  a  physical  impossibility,  and  he  questioned 
him  closely  as  to  the  secret  of  his  astonishing  rapidity 
of  execution,  for  he  never  required  more  than  one  sit- 
ting from  his  patrons.  Wigan  states  that  he  saw  him 
paint  a  miniature  of  a  well-known  personage  in  eight 
hours,  which  was  incomparable  in  its  fidelity  to  nature 
and  finished  execution.  Wigan  asked  him  to  give  him 
some  details  of  the  method  he  adopted,  and  he  gave  him 
the  following  answer  :  "  When  a  sitter  presents  himself, 
I  look  at  him  attentively  for  half  an  hour,  sketching 
the  outlines  of  his  features  on  my  canvass  during  the 


THE  INFLUENCE  OF  THE  IMAGINATION.  63 


time.  I  have  no  occasion  for  a  longer  sitting,  and  I 
pass  on  to  some  one  else.  When  I  wish  to  continue 
the  first  portrait,  I  take  the  sitter  in  my  imagination, 
and  I  seat  him  in  the  chair,  where  I  see  him  as  dis- 
tinctly as  if  he  were  really  there,  and  I  can  even 
heighten  a  tint,  or  soften  down  a  clumsy  form  at  will, 
without  altering  the  likeness.  I  look  from  time  to  time 
at  the  imaginary  figure,  and  I  go  on  painting.  I  stop 
now  and  then  to  examine  his  position,  absolutely  as  if 
the  original  were  before  me ;  for  every  time  I  look 
towards  the  chair  I  see  the  sitter.  This  method  of  pro- 
ceeding has  rendered  me  very  popular ;  and  as  I  have 
always  succeeded  in  catching  the  likeness  of  my  pa- 
trons, they  have  been  simply  enchanted  at  my  sparing 
them  the  tedious  sittings  exacted  by  other  painters. 
Little  by  little  I  have  begun  to  lose  the  distinction  be- 
tween the  real  and  imaginary  sitter,  and  I  have  often 
maintained  stoutly  that  my  patrons  had  already  sat  to 
me  on  the  previous  day.  At  last  I  became  convinced 
that  it  was  the  real  sitters  that  I  saw,  and  thenceforth 
all  became  confusion.  I  suppose  my  friends  took  alarm 
at  my  hallucinations,  for  I  remember  nothing  of  what 
happened  during  the  thirty  years  that  I  remained  in  the 
madhouse.  This  long  period  has  left  no  trace  on  my 
memory,  except  indeed  the  last  six  months  of  my  con- 
finement. It  seems  to  me,  however,  that  when  my 
friends  talk  of  having  visited  me  I  have  some  vague  re- 
collection of  the  fact ;  but  it  is  a  subject  that  I  do  not 
care  to  pursue.' ' 

The  most  remarkable  feature  of  the  case  is,  that  this 
artist  after  a  lapse  of  thirty  years  resumed  his  pencil, 
and  painted  almost  as  well  as  when  he  was  forced  by 
madness  to  abandon  his  art. 

This  faculty  of  being  able  to  evoke  shadows,  with 
which  to  people  one's  solitude,  may  be  carried  so  far  as 
to  transform  real  persons  into  phantoms.  Hyacinth 


64 


TI1E  WONDERS  OF  OPTICS. 


Langlois,  a  distinguished  artist,  living  at  Rouen,  tells 
us  that  Talma,  with  whom  he  was  extremely  intimate, 
confided  to  him  that,  whenever  he  went  upon  the  stage, 
he  had  the  power,  by  mere  force  of  will,  to  cause  the 
clothes  and  flesh  of  his  numerous  auditory  to  disappear, 
and  become  transformed  from  living  beings  into  so  many 
skeletons.  When  his  imagination  had  peopled  the  house 
with  these  singular  phantoms,  the  emotion  he  felt  was 
so  great  that  it  gave  his  dramatic  powers  still  greater 
force,  and  enabled  him  to  produce  the  wonderful  effects 
that  have  made  his  name  so  famous. 

Wigan  says,  that  he  o  ce  knew  a  most  intelligent  and 
amiable  man,  who  could  at  will  evoke  his  own  image. 
He  often  laughed  at  seeing  his  second  self  standing  be- 
fore him,  the  phantom  appearing  to  laugh  as  heartily  as 
himself.  This  illusion  wras  for  a  long  time  a  matter  of 
amusement  to  him,  but  at  last  he  became  persuaded 
that  he  was  haunted  by  his  own  double.  His  second 
self  appeared  to  hold  arguments  with  him  continually, 
and  beat  him  frequently  on  various  points  of  dispute,  a 
matter  which  mortified  him  excessively,  as  he  was  rather 
proud  of  his  powers  of  reasoning.  This  gentleman, 
although  always  considered  as  being  somewhat  eccen- 
tric, wTas  never  put  under  the  slightest  restraint,  and  at 
last  the  creature  of  his  imagination  so  tormented  him, 
that  he  resolved  not  to  live  through  another  year.  He 
consequently  paid  all  his  debts,  arranged  his  affairs,  and 
waited  pistol  in  hand  until  the  clock  struck  twelve  on 
the  31st  of  December,  and  then  deliberately  blew  out 
his  brains. 

In  Abercromby  on  the  Mind  we  read  an  account  01 
the  observations  made  by  a  gentleman  who  was  the  vic- 
tim of  illusions  during  the  whole  of  a  pretty  long  life. 
If  he  met  a  friend  in  the  street,  he  was  urable  to  tell 
at  first  whether  he  saw  a  real  human  being  or  only  a 
phantom.    By  close  examination  he  could  detect  a  dif- 


THE  INFLUENCE  OF  THE  IMAGINATION. 


65 


ference  between  the  real  person  and  the  creature  of  his 
imagination,  the  features  of  the  former  being  sharper 
and  more  defined  than  those  of  the  phantom ;  but  in 
general  he  was  obliged  to  test  the  reality  of  the  figure 
he  saw  by  the  senses  of  touch  and  hearing.  He  was 
able,  by  concentrating  his  thoughts  upon  the  appear- 
ance of  any  friend,  to  call  up  his  image  ;  a  power  which 
extended  even  to  scenes  that  he  had  witnessed.  Although 
he  could  produce  these  hallucinations  at  will,  he  was 
powerless  in  making  them  disappear ;  and  when  once 
he  succeeded  in  calling  forth  these  creatures  of  his  ima- 
gination, he  never  could  tell  how  long  the  delusion 
would  last.  This  gentleman  was  in  the  prime  of  life,  a 
good  man  of  business,  and  otherwise  in  a  perfect  state 
of  mental  and  bodily  health.  A  member  of  his  family 
possessed  the  same  faculty,  but  in  a  minor  degree. 

In  1806,  General  Rapp,  when  returning  from  the 
siege  of  Dantzic,  having  occasion  to  speak  to  the  Em- 
peror Napoleon,  walked  into  his  private  room  without 
being  announced,  and  found  him  in  such  a  profound 
state  of  abstraction,  that  he  remained  for  some  time 
unperceived  by  his  imperial  master.  The  General,  see- 
ing him  thus  perfectly  motionless,  fancied  he  must  be 
ill,  and  purposely  made  a  slight  noise.  Napoleon  in- 
stantly turned  his  head,  seized  the  General  by  the  arm, 
and  pointing  upwards,  exclaimed,  "  Do  you  see  it  up 
there?"  The  General,  hardly  knowing  what  to  say, 
remained  silent ;  but  the  Emperor  repeated  his  ques- 
tion, and  he  was  obliged  to  reply,  that  he  saw  nothing. 
"  What,"  said  the  Emperor,  "you  don't  see  anything? 
You  don't  see  my  star  shining  before  your  eyes  ?"  And 
becoming  more  and  more  animated,  he  went  on  to  say, 
that  the  mysterious  visitor  had  never  abandoned  him, 
that  he  saw  it  throughout  all  his  great  battles,  that  it 
always  led  him  onward,  and  that  he  was  never  happy 
but  when  he  was  gazing  at  it. 

E 


66 


THE  WONDERS  OF  OPTICS. 


That  such  hallucinations  have  no  real  existence  as  far 
as  the  eye  goes,  is  proved  by  the  fact  of  many  people 
who  have  lost  their  sight,  being  subject  to  them.  It  is 
hardly  to  be  wondered  at  that  those  who  by  accident 
have  been  deprived  of  their  sight,  should  wish  so 
ardently  to  see  once  more  the  persons  and  sights  they 
have  taken  pleasure  in,  that  they  should  at  last  create 
for  themselves  illusions  of  this  character.  The  same 
thing  has  frequently  occurred  with  those  whose  sight  is 
more  or  less  weak.  An  old  man  of  eighty,  who  was 
purblind,  never  sat  down  to  a  table  during  the  last 
years  of  his  life,  without  seeing  around  him  a  number 
of  his  friends  who  had  long  been  dead,  dressed  in  the 
costume  of  fifty  years  before.  This  old  man  had  but 
one  eye,  which  was  extremely  weak,  and  wore  a  pair  of 
green  preservers,  in  the  glass  of  which  he  continually 
saw  his  own  face  reflected. 

Doctor  Dewar,  of  Stirling,  mentioned  to  Abercromby 
a  very  remarkable  instance  of  this  species  of  hallucina- 
tion. The  patient,  who  was  quite  blind,  never  walked 
in  the  street  without  seeing  a  little  old  woman  hobbling 
on  before  him  and  leaning  on  a  stick.  This  apparition 
always  disappeared  when  he  entered  his  house. 

Similar  illusions  frequently  happen  to  every  one,  even 
the  most  healthy  amongst  us,  but  a  little  consideration 
soon  puts  them  to  flight.  It  would  be  useless  to  men- 
tion the  numberless  cases  in  which  a  square  tower  has 
appeared  round,  or  where  the  landscape  has  suddenly 
seemed  to  recede  from  the  sight.  Such  illusions  as 
these  have  been  long  well  known,  and  appreciated  at 
their  proper  value ;  but  there  are  others  whose  true 
cause  has  remained  a  mystery,  until  explained  by  the 
progress  of  science,  such  as  the  Spectre  of  the  Brocken, 
the  Fata  Morgana,  and  the  mirage. 

Analogous  appearances  have  been  seen  in  Westmore- 
land and  other  mountainous  districts,  the  inhabitants 


THE  INFLUENCE  OF  THE  IMAGINATION.  67 


imagining  that  the  air  was  full  of  troops  of  cavalry,  and 
whole  armies  even  ;  such  illusions  resulting  simply  from 
the  shadows  of  men  and  horses  passing  over  an  oppo- 
site mountain  being  thrown  on  the  fog. 

A  vast  number  of  different  circumstances  give  rise  to 
these  illusions,  such  as  a  strong  impression,  or  the  re- 
collection of  some  striking  event,  which  may  easily 
cause  them,  by  the  association  of  ideas.  Wigan  relates, 
that  being  at  a  soiree  held  at  the  house  of  M.  Bellart, 
a  few  days  after  the  execution  of  Marshal  Ney,  the 
groom  of  the  chamber,  instead  of  calling  out  the  name 
of  M.  Marechal  aine  (M.  Marechal,  senior),  announced 
the  arrival  of  M.  le  Marechal  Ney.  A  shudder  passed 
through  the  company,  and  many  of  them  declared,  that 
for  an  instant  they  saw  the  face  and  figure  of  the  dead 
man  in  place  of  those  of  his  involuntary  representative. 

When  the  mind  is  thus  prepared,  the  most  familiar 
objects  are  transformed  into  phantoms.  Ellis  relates 
an  anecdote  of  this  kind,  which  he  heard  Irom  an  eye- 
witness, who  was  a  ship's  captain  of  Newcastle- on-Tyne. 
During  a  voyage  that  he  made,  the  ship's  cook  died. 
Some  days  after  the  funeral,  the  chief  mate  came  run- 
ning to  him  in  a  great  fright,  with  the  news  that  the 
ship's  cook  was  walking  on  the  water,  astern  of  the 
vessel,  and  that  all  the  crew  were  on  deck  looking  at 
him.  The  captain,  who  was  angry  at  being  disturbed 
with  so  nonsensical  a  tale,  answered  sharply,  that  they 
had  better  put  the  ship  about  and  race  the  ghost  to 
Newcastle.  His  curiosity,  however,  was  presently 
aroused,  and  he  went  upon  deck  and  looked  at  the  spec- 
tre. Be  frankly  avowed  that  for  some  moments  he  saw 
what  really  appeared  to  be  his  old  shipmate,  just  as  he 
knew  him  in  life,  with  his  walk,  clothes,  cap  and  figure 
perfectly  resembling  those  of  the  dead  man.  The  panic 
became  general,  and  every  one  was  struck  motionless 
for  a  time.    He  had  the  presence  of  mind,  however,  to 


68 


THE  WONDERS  OF  OPTICS. 


seize  the  helm  and  put  the  ship  about,  when  as  they 
neared  the  ghost,  they  found  the  absurd  cause  of  their 
fright  was  a  broken  mast  from  some  wreck,  which  was 
floating  after  them  in  an  upright  position.  If  the  cap- 
tain had  not  boldly  sailed  up  to  the  supposed  ghost,  the 
story  of  the  dead  cook  walking  upon  the  water  would 
have  continued  to  this  day  to  terrify  half  the  good  in- 
habitants of  Newcastle. 

Such  facts  as  these  are  innumerable,  and  we  shall 
mention  a  few  more  which  will  explain  a  host  of  stories 
found  in  various  ancient  and  modern  authors. 

Ajax  was  so  angry  at  the  arms  of  Achilles  being 
awarded  to  Ulysses,  that  he  became  furious,  and,  seeing 
a  herd  of  pigs,  drew  his  sword  and  fell  upon  them, 
taking  them  for  Greeks.  He  next  seized  a  couple  of 
them  and  beat  them  cruelly,  loading  them  at  the  same 
time  with  insults,  imagining  one  of  them  to  be  Aga- 
memnon, his  judge,  and  the  other  Ulysses,  his  enemy. 
When  he  came  to  himself,  he  was  so  ashamed  at  what 
he  had  done,  that  he  stabbed  himself  with  his  sword. 

Theodoric,  blinded  by  jealousy  and  yielding  to  the 
base  solicitations  of  his  courtiers,  ordered  that  Symma- 
ehus,  one  of  the  most  upright  men  of  his  time,  should 
be  put  to  death.  The  cruel  order  had  hardly  been  ex- 
ecuted, when  the  king  was  seized  with  remorse,  and 
bitterly  reproached  himself  with  his  crime.  One  day  a 
new  kind  of  fish  was  put  upon  the  table,  when  the  king 
suddenly  cried  out  that  he  saw  in  the  head  of  the  fish 
the  absolute  resemblance  of  that  of  his  victim.  This 
vision  had  the  effect  of  plunging  the  king  into  a  state 
of  melancholy  that  lasted  his  whole  life. 

Bessus  once,  when  surrounded  by  his  guests  and  giv- 
ing himself  up  to  the  enjoyment  of  the  feast,  ceased 
suddenly  to  listen  to  the  flattering  speeches  of  his  cour- 
tiers. He  apparently  listened  with  great  attention  to 
some  sound  that  was  heard  by  no  one  else,  and  suddenly 


THE  INFLUENCE  OF  THE  IMAGINATION. 


69 


leaping  from  his  couch,  mad  with  rage,  he  seized  his 
sword  and  rushing  at  a  swallow's  nest  that  was  near, 
beat  it  down,  killing  the  poor  birds  inside  it,  crying  out 
that  these  insolent  birds  dared  to  reproach  him  with  the 
murder  of  his  father.  Surprised  at  such  a  sight,  his 
courtiers  gradually  disappeared,  and  it  became  known 
some  time  afterwards  that  Bessus  was  really  guilty,  and 
that  the  senseless  action  he  had  performed  simply  re- 
sulted from  the  voice  of  conscience. 

The  illusions  of  sight  and  hearing  are  often  found  to 
take  an  epidemic  form,  and  historians  relate  an  immense 
number  of  anecdotes  bearing  on  this  particular  phase  of 
self-delusion.  One  of  the  commonest  of  them  is  that 
which  transforms  the  clouds  into  armies  and  figures  of 
all  kinds.  Religious  prejudices,  optical  phenomena, 
physical  laws  that  are  still  unknown,  dangerous  fevers, 
derangements  of  the  brain,  afford  a  natural  explanation 
of  these  hallucinations. 

We  have  borrowed  most  of  these  examples  from 
Briere  de  Boismont's  works,  for  the  special  purpose  of 
showing  how  easy  it  is  to  deceive  the  imagination,  and 
to  demonstrate  the  facility  with  which  the  sense  of  sight 
is  led  astray  without  the  intervention  of  complicated 
apparatus.  In  addition,  we  may  quote  instances  from 
Brewster,  showing  the  ease  with  which  the  imagination 
enables  us  to  see  distinct  forms  in  a  confused  mass  of 
flames,  or  in  a  number  of  shadows  superposed  upon  each 
other.  This  great  philosopher  gives  us  an  anecdote  of 
Peter  Heamann,  a  Swedish  pirate  and  murderer.  One 
day  that  his  crew  were  repairing  some  unimportant 
portion  of  the  ship,  after  having  pitched  the  place  well 
he  took  the  brush  in  order  to  tar  the  other  parts  of 
the  vessel,  which  were  much  in  want  of  such  treatment; 
but  as  soon  as  he  spread  the  pitch  over  the  timbers  of 
the  ship,  he  was  thunderstruck  at  seeing  apparently 
reflected  in  its  shining  surface  the  image  of  a  gallows 


70 


THE  WONDERS  OF  OPTICS. 


with  a  headless  man  beneath.  The  head  belonging  to 
the  body  was  lying  before  it,  and  the  body  itself  was 
depicted  with  every  limb — legs,  thighs,  and  arms — 
perfect.  He  frequently  told  his  crew  of  these  illusions, 
adding  that  it  was  evidently  a  prediction  of  the  fate  in 
store  for  them.  He  was  often  in  such  a  state  of  terror, 
that  on  calm  days  he  would  drop  down  into  the  hold 
and  wrap  himself  up  in  a  spare  sail  in  order  not  to 
catch  sight  of  the  horrible  image  that  he  constantly  saw 
in  the  shining  surface  of  the  tar. 

The  imagination  really  seems  to  create  for  itself  a  sort 
of  mental  visual  organ  which  is  in  intimate  relation 
with  that  of  the  body,  and  which  often  takes  its  place 
so  efficiently — as  in  the  case  of  dreams — that  the  mind 
is  utterly  unable  to  perceive  the  substitution.  It  is  on 
account  of  this  that  practical  opticians  are  so  unsparing 
in  their  endeavours  to  p/edispose  their  spectators  to 
being  deceived. 

When  both  the  body  aid  mind  are  healthy,  the  re- 
lative intensity  of  the  two  kinds  of  impressions  is  very 
unequally  divided,  mental  images  being  more  evanescent 
and  comparatively  weak,  and  with  persons  of  ordinary 
temperament  incapable  of  effacing  or  disturbing  the 
reflections  of  visible  oSjects.  The  affairs  of  life  c  >uld 
not  go  on  if  the  memory  introduced  amongst  them  bril- 
liant representations  of  the  past  in  the  midst  of  ordinary 
domestic  scones  or  the  objects  familiar  to  us.  We  may 
accou  it  for  this  by  supposing  that  the  set  of  nerves 
which  carries  the  efforts  of  the  memory  to  the  brain 
cannot  execute  their  functions  at  the  same  time  as  those 
which  take  cognizance  of  the  images  reflected  on  the 
retina.  In  other  words,  the  mind  cannot  accomplish 
two  separate  functions  at  one  and  the  same  time,  and 
the  mere  act  of  directing  the  attention  to  one  class  of 
subjects  causes  all  others  to  become  instantly  imper- 
ceptible.   The  exercise  of  the  mind  in  these  instances  is, 


THE  INFLUENCE  OF  THE  IMAGINATION.  71 


however,  so  rapid  that  the  alternate  appearance  and 
disappearance  of  the  two  different  impressions  is  com- 
pletely unnoticed.  Thus,  for  instance,  while  looking  at 
the  dome  of  St.  Paul's,  if  our  memory  suddenly  evokes 
the  image  of  some  other  object,  Mont  Blanc  for  instance, 
the  picture  of  the  cathedral,  although  still  depicted  on 
our  retina,  is  momentarily  effaced  by  the  effort  of  the 
will,  although  we  may  not  change  the  position  of  our 
eyes  during  the  time.  While  the  memory  continues 
to  dwell  on  the  picture  it  has  called  up,  it  is  seen 
with  sufficient  distinctness,  although  its  details  may 
be  somewhat  misty  and  its  colours  confused ;  but  as 
soon  as  the  wish  to  see  it  passes  away  the  whole 
disappears,  and  the  cathedral  is  seen  in  all  its  former 
distinctness. 

In  darkness  and  solitude,  wThen  surrounding  objects 
produce  no  images  that  can  interfere  with  those  of  the 
mind,  these  latter  are  more  lively  and  distinct :  and 
when  in  addition  we  are  half  asleep  and  half  awake, 
the  intensity  of  mental  impressions  approaches  that  of 
visible  objects.  In  the  case  of  persons  of  studious 
habits  who  are  continually  employed  in  mental  effort, 
these  images  are  more  distinct  than  with  those  who 
follow  the  ordinary  avocations  of  life,  and  during  their 
working  hours  rarely  seethe  objects  round  them.  The 
earnest  thinker,  absorbed  by  meditation,  is  in  a  manner 
deprived  for  the  time  of  the  use  of  his  senses.  His 
children  and  servants  pass  in  and  out  of  his  study 
without  his  seeing  them,  they  speak  to  him  without 
his  hearing  them  and  they  may  even  try  to  rouse 
him  from  his  reverie  without  success;  and  yet  his 
eyes,  ears,  and  nerves  received  the  impression  of  light, 
sound,  and  touch.  In  such  instances,  the  mind  of  the 
philosopher  is  voluntarily  occupied  in  following  out  an 
idea  which  interests  him  profoundly  ;  but  even  the  most 
unlearned  and  thoughtless  of  us  sees  the  images  of  dead 


72 


THE  WONDERS  OF  OPTICS. 


or  absent  friends  with  his  mind's  eye,  or  even  fantastic 
figures  which  have  nothing  to  do  with  the  train  of 
thought  he  may  be  pursuing.  It  is  with  these  involun- 
tary apparitions  as  with  spectres  of  the  imagination : 
although  they  are  intimately  connected  with  some 
thought  that  has  passed  through  our  mind  unperceived, 
it  is  impossible  to  trace  a  single  link  of  the  chain  con- 
necting them  together. 


WHAT  IS  LIGHT? 


73 


PART  II 
THE  LAWS  OF  LIGHT. 


CHAPTER  I. 

WHAT  IS  LIGHT? 

Everybody  knows  the  effects  of  the  action  of  light, 
without,  however,  understanding  precisely  what  consti- 
tutes light  itself.  Any  formal  definition  would  rather 
puzzle  than  help  the  student ;  we  must  therefore  con- 
tent ourselves  with  saying  that  light  is  that  effect  of 
force  which  causes  us  to  perceive  external  objects. 

A  man  who  was  blind  from  his  birth,  and  upon  whom 
the  operation  for  cataract  had  been  successfully  per- 
formed, had  accustomed  himself  for  a  long  time  to  ima- 
gine the  nature  of  those  unknown  phenomena  that  his 
affliction  had  prevented  him  from  observing.  He  had 
arranged  in  his  mind  the  various  definitions  that  had 
been  given  to  him  as  to  the  nature  of  light,  and  having 
combined  them,  he  fancied  he  had  acquired  some  notion 
of  what  the  sense  of  vision  really  meant.  But  what 
was  the  astonishment  of  the  surgeon  who  had  restored 
to  him  his  fifth  sense,  when  he  asked  him  to  give  his 
opinion  upon  the  effects  of  light,  to  see  him  take  up  a 
lump  of  sugar  and  reply  that  it  was  under  that  form 
that  he  had  imagined  it  to  himself. 


71 


THE  WONDERS  OF  OPTICS. 


As  for  us  who  have  the  happiness  of  possessing  the 
sense  of  sight,  we  know  this  mysterious  agent  more  by 
the  enjoyment  that  we  have  derived  from  it,  than  from 
any  analysis  we  have  made  of  its  nature.  It  is  an  end- 
less chain  that  connects  us  with  the  entire  universe  ;  a 
bond  that  laughs  at  distance  and  spans  the  abysses  of 
space.  By  means  of  light  we  can  appreciate  the  beau- 
ties of  hue  and  form,  and  by  its  power  we  touch  as  it 
were  the  inaccessible.  It  constitutes  the  most  intimate 
connexion  between  ourselves  and  external  objects — a 
connexion  that  seems  even  to  alter  our  temper,  disposi- 
tion, and  character,  according  to  the  variations  of  its 
intensity.  The  dull  and  foggy  days  of  winter,  those 
days  when  sleet  and  rain  struggle  in  the  atmosphere, 
spr.ead  like  a  veil  over  us,  and  throw  a  shadow  upon 
our  life.  The  return  of  the  bright  spring  sun,  the  re- 
appearance of  light  and  blue  sky,  on  the  contrary,  open 
up  our  hearts  and  minds,  gay  nature  enchants  us  once 
more,  and  a  feeling  of  fresh  happiness  prepares  us  for 
the  coming  glories  of  the  newly  risen  year. 

This  intimate  connexion  between  the  light  of  heaven 
and  the  human  mind,  hallowed  as  it  is  by  our  desire  to 
rise  towards  the  Source  of  all  light,  might  be  made  the 
subject  of  many  eloquent  pages  ;  and  it  would  be  an 
interesting  and  useful  task  to  show  the  gradual  pro- 
gress of  mankind  from  those  ancient  people  who  trem- 
bled at  the  approach  of  darkness,  and  who  fervently 
saluted  the  dawn  with  prayers  and  praises,  dowTn  to  the 
philosophers  of  the  present  age,  w7ho  investigate  its 
effects  with  so  much  reverential  joy.  But  wc  must 
cease  paying  any  more  attention  to  the  superficial  ac- 
tion of  this  marvellous  force  which  in  these  latter  days 
has  become,  in  the  hands  of  man,  the  source  of  so 
many  illusions  and  the  origin  of  a  complete  world  of 
rich  and  brilliant  pictures,  but  which  after  all  only  exist 
in  the  imagination. 


WHAT  IS  LIGHT? 


75 


It  was  believed  for  a  long  time  that  light  was  a  com- 
pact mass  of  tiny  particles  emitted  by  luminous  bodies, 
which  struck  our  eyes  and  so  produced  the  phenomenon 
of  vision.  These  particles  or  molecules  were  naturally 
thought  to  be  extremely  minute,  and  the  objects  illu- 
minated by  them  were  supposed  to  throw  them  off  as 
if  they  were  endowed  with  elasticity.  Under  this  hy- 
pothesis, light  was  a  material  body.  The  illustrious 
Newton  was  the  first  propagator  of  this  theory  ;  the 
last  was  M.  Biot,  a  French  philosopher,  lately  dead. 

The  undulatory  theory  has  now-a-days  completely 
superseded  the  corpuscular  hypothesis.  It  was  first 
started  about  the  year  1660  by  the  Dutch  philosopher 
Huyghens,  who  has  left  behind  him  numerous  treatises 
on  optics,  and  the  properties  of  light,  as  well  as  a  curi- 
ous account  of  the  inhabitants  of  the  other  members  of 
the  solar  system,  including  a  minute  description  of  the 
various  planetary  manners  and  customs.  At  the  begin- 
ning of  the  present  century,  Fresnel  showed,  by  the 
most  brilliant  discoveries  the  superiority  of  this  theory, 
and  shortly  after  Arago  confirmed  him  in  his  demon- 
strations. According  to  the  undulatory  hypothesis, 
light  is  not  a  mass  of  molecules  emitted  by  a  luminous 
bo  ly,  but  simply  the  vibration  of  an  elastic  fluid  which 
is  conceived  to  fill  the  whole  of  space.  A  comparative 
example  may  assist  you  in  understanding  this  theory 
more  clearly.  If  you  throw  a  stone  into  a  smooth 
piece  of  water,  there  will  form  around  the  point  where 
the  stone  fell,  a  series  of  circular  undulations,  starting 
from  the  centre  and  gradually  enlarging  themselves. 
If  a  loud  noise  is  suddenly  heard,  the  same  effect  is 
produce  1  round  the  point  from  whence  the  sound  pro- 
ceeds. A  series  of  waves  are  formed  which  spread  not 
only  h>riz  mtally,  as  on  the  surface  of  the  water  dis- 
turbed by  the  stone,  but  in  every  direction.  In  fact, 
in  the  case  of  sounds,  the  waves  are  so  many  gradually 


76 


THE  WONDERS  OF  OPTICS. 


increasing  spheres.  In  the  case  of  light,  when  a  lumi- 
nous body  is  placed  in  space,  the  ether  which  surrounds 
it  is  thrown  into  a  state  of  vibration,  and  the  motion  is 
immediately  propagated  in  all  directions,  with  extreme 
velocity.  It  is  these  undulations  that  produce  upon  our 
eyes  the  sensation  of  light.  We  may  therefore  say  that 
light,  like  sound,  is  movement,  while  darkness,  like  si- 
lence, is  absolute  rest. 

Many  people  still  believe  that  light  is  propagated  in- 
stantaneously, and  cannot  bring  themselves  to  imagine 
th^t  we  do  not  see  a  flame  the  moment  we  light  it,  but 
only  an  instant  after.  I  have  myself  spoken  to  well- 
educated  people  possessed  of  good  judgment  and  a  cer- 
tain amount  of  elemen  ary  knowledge,  who  could  never 
bring  themselves  to  believe  that  we  see  the  stars,  not  as 
they  now  exist,  but  as  they  appeared  at  the  particular 
moment  when  the  luminous  wave  by  which  we  are  ena- 
bled to  perceive  them  left  their  surface,  and  which  only 
reaches  us  after  travelling  through  space  a  certain  num- 
ber of  years,  days,  or  hours,  according  to  their  distance. 
It  is  extremely  useful  and  interesting  to  form  a  correct 
idea  upon  the  way  in  which  light  is  propagated. 

The  determination  of  the  prodigious  quickness  with 
which  the  waves  of  light  move  through  space,  says 
Arago,  is  undoubtedly  one  of  the  happiest  results  of 
modern  astronomy.  The  ancients  believed  that  it  moved 
with  infinite  velocity,  and  their  viewr  of  the  subject  wTas 
not,  like  so  many  of  the  questions  relating  to  physics,  a 
mere  opinion  without  proof;  for  Aristotle,  in  mention- 
ing it,  brings  forward  the  apparently  instantaneous 
transmission  of  daylight.  This  notion  was  disputed  by 
Alhazen,  in  his  Treatise  on  Optics,  but  only  by  meta- 
physical weapons,  which  were  again  opposed  by  several 
very  worthless  arguments,  by  his  commentator,  Porta, 
although  he  admitted  the  immateriality  of  light.  Galileo 
seems  to  have  been  the  first  amongst  modern  philoso- 


WHAT  IS  LIGHT  ?  77 

phers  who  endeavoured  to  determine  the  velocity  of 
light  by  experiment.  In  the  first  of  his  dialogues,  Delle 
Scienze  Nuove,  he  announces  by  the  mouth  of  Salviati. 
one  of  the  speakers  present,  the  ingenious  means  he 
had  employed,  and  which  he  thought  quite  sufficient  to 
solve  the  question.  Two  observers  with  lights  were 
placed  at  the  distance  of  one  mile  from  each  other;  one 
of  them  extinguished  his  light,  and  the  other  as  soon  as 
he  perceived  it  extinguished  his.  But  as  the  first  ob- 
server saw  the  second  light  disappear  the  instant  he 
had  extinguished  his  own,  Galileo  concluded  that  light 
was  propagated  instantaneously  through  a  distance  dou- 
ble that  which  separated  the  two  observers.  Certain 
analogous  experiments  that  were  made  by  the  members 
of  the  Academy  Del  Cimento,  but  at  three  times  the 
distance,  led  to  precisely  the  same  conclusions. 

These  attempted  proofs  seem  at  first  sight  to  be  ab- 
surd, when  we  think  of  the  vastness  of  the  problem  to 
be  solved ;  but  we  must  judge  these  experiments  with 
less  severity,  when  we  consider  that  almost  at  the  same 
epoch,  men  of  such  well-deserved  repute  as  Lord  Bacon 
believed  that  the  velocity  of  light,  like  that  of  sound, 
was  sensibly  altered  by  the  force  and  direction  of  the 
wind. 

Descartes,  whose  theories  upon  light  had  so  much 
analogy  with  those  known  under  the  name  of  the  undu- 
latory  hypothesis,  believed  that  light  was  transmitted 
instantaneously  throughout  any  distance,  and  endea- 
vours to  prove  his  position  by  proofs  that  he  thought  he 
had  obtained  whilst  observing  an  eclipse  of  the  moon. 
It  must  be  acknowledged,  however,  that  his  very  inge- 
nious train  of  reasoning  proves  that  whether  the  trans- 
mission of  light  is  instantaneous  or  not,  it  is  at  least  too 
considerable  to  be  determined  by  experiments  made  on 
the  earth,  like  those  of  Galileo,  and  which  he  vainly 
hoped  would  have  solved  the  question. 


78 


THE  WONDEKS  OF  OPTICS, 


The  frequent  occupations  of  the  first  satellite  of  Ju- 
piter, the  discovery  of  which  was  almost  consequent 
upon  that  of  lenses,  furnished  Romer  with  the  first 
means  of  demonstrating  that  light  was  propagated  by 
perceptible  degrees. 

In  tracing  out  the  history  of  human  knowledge,  says 
Dr.  Lardner,  we  have  frequently  to  point  out  with  some 
little  surprise,  joined  to  a  feeling  of  profound  humility, 
the  important  part  played  by  chance  in  the  advancement 
of  science.  In  searching  zealously  after  mere  trifles 
which,  wlion  found,  are  of  no  consequence,  we  frequently 
lay  our  hands  on  inestimable  treasures.  The  frequency 
of  this  fact  impresses  the  mind  with  the  notion  that  some 
secret  and  unceasing  powrer  exists,  in  accordance  with 
which  human  knowledge  and  science  are  continually 
progressing.  It  is  in  physical,  as  in  moral  philosophy. 
In  our  ignorance — like  the  dog  mentioned  by  iEsop, 
which,  seeing  in  the  water  the  reflection  of  the  prey  it 
held  in  its  mouth,  dropped  the  substance  and  tried  to 
seize  the  shadow — we  are  continually  searching  after 
trifles ;  but,  more  fortunate  than  the  animal  of  whom 
we  have  been  speaking,  the  shadow  that  we  try  to  seize 
is  often  transformed  into  a  rich  treasure.  We  can  say 
with  every  co  fidence  that  u  the  Providence  which 
shapes  our  ends,"  knows  our  wants  better  than  we  do 
ourselves,  and  bestows  on  us  the  things  we  ought  to  have 
asked  for  instead  of  tho  se  we  have  asked  for.  We  shall 
find  a  very  simple  proof  of  this  in  the  history  of  the 
discovery  of  the  velocity  of  light. 

A  short  time  after  the  invention  of  the  telescope  and 
the  consequent  discovery  of  Jupiter's  satellites,  Romer, 
a  celebrated  Danish  astronomer,  was  engaged  in  a  se- 
ries of  observations,  the  object  of  which  wras  to  deter- 
mine the  time  which  one  of  these  bodies  took  to  revolve 
round  its  planet.  The  method  employed  by  Romer  was 
to  observe  the  successive  oc  ultations  of  the  satellite, 


WHAT  IS  LIGHT? 


79 


and  to  notice  the  interval  that  elapsed  between  each  of 
them.  But  it  at  last  happened  that  the  interval  be- 
tween the  two  occupations,  which  was  about  forty-five 
hours,  became  prolonged  by  periods  of  8,  13,  and  16 
minutes,  during  that  half  of  the  year  when  the  earth 
Tvas  receding  from  the  planet,  while  it  became  propor- 
tionally cut  short  during  the  rest  of  the  year.  Romer 
was  struck  by  a  happy  idea ;  he  suspected  instantly 
that  the  moment  when  he  remarked  the  disappearance 
of  the  satellite  was  not  always  coincident  with  the  in- 
stant when  it  really  took  place,  but  that  it  sometimes 
appeared  to  happen  later — that  is  to  say,  after  an  in- 
terval of  time  sufficiently  long  to  allow  the  light  that 
had  left  the  satellite  immediately  after  its  disappear- 
ance, to  reach  the  eye  of  the  observer.  Hence  it  be- 
came evident  that  the  farther  oft'  the  earth  was  from  the 
satellite,  the  longer  was  the  interval  of  time  between  its 
disappearance  and  that  of  the  arrival  of  the  last  portions 
of  its  light  upon  the  earth  ;  but  that  the  mcment  of  the 
disappearance  of  the  satellite  is  that  of  the  commence- 
ment of  the  occultation,  and  that  the  ircment  of  the 
arrival  of  the  last  portions  of  light  is  tl  at  when  the 
commencement  of  the  occultation  is  observed. 

It  was  thus  that  Rom er  explained  the  difference  be- 
tween the  calculated  and  observed  time  of  the  occulta- 
tion, and  he  saw  that  he  was  on  the  threshold  of  a 
great  discovery.  In  a  word,  he  saw  that  light  propa- 
gated itself  through  space  with  a  certain  velocity,  and 
that  the  fact  we  have  just  mentioned  furnished  the  pre- 
cise means  of  measuring  it. 

Thus  the  occultation  of  the  satellite  was  retarded  one 
second  for  every  185,000  miles  that  the  earth  is 
distant  from  Jupiter  ;  the  reason  being,  that  a  ray  of 
light  takes  a  second  to  travel  this  distance,  or,  in  other 
words,  because  the  velocity  of  light  is  at  the  rate  of 
185,000  miles  per  second. 


80 


THE  WONDERS  OF  OPTICS. 


It  must  be  remembered  when  considering  this  sub- 
ject, that  in  any  system  of  undulations  or  vibrations,  no 
matter  through  what  medium  they  are  propagated,  their 
movement  is  simply  a  change  of  form,  and  not  a  trans- 
mission of  matter.  The  waves  which  spread  round  a 
central  point  when  a  stone  is  thrown  into  the  water, 
give  one  the  idea  that  the  water  which  forms  the  wave 
really  moves  towards  the  observer.  But  it  is  not  so,  as 
may  be  readily  proved  by  placing  on  the  surface  a  float- 
ing body,  which  we  shall  find  is  but  little,  if  at  all,  influ- 
enced by  the  undulations  of  the  water.  The  appearance 
of  rolling  waves  given  on  the  stage  by  means  of  a  painted 
cloth,  to  which  an  undulatory  motion  is  given,  is  an  in- 
stance of  this  apparent  movement.  In  the  case  of  the 
floating  body,  which  would  follow  the  movements  of  the 
water,  we  shall  find  that  wave  after  wave  rolls  to  the 
shore,  in  the  same  way  as  the  painted  marks  on  the 
imitation  sea  keep  their  place,  although  the  cloth  itself 
undulates.  The  waves  of  the  sea  even  appear  to  the  eye 
to  be  endowed  with  a  progressive  motion,  but  an  instant's 
observation  will  convince  us  of  our  error;  for  if  such 
were  the  case,  every  object  floating  on  the  ocean  would 
be  gradually  carried  on  shore.  A  vessel  floating  on  the 
waves  is  not  carried  along  by  them,  at  least  not  until  it 
reaches  within  a  few  yards  of  the  shore,  where  the 
water  is  really  in  motion  ;  but  out  in  the  open  sea  a 
floating  body  will  alternately  rise  on  their  crests,  and 
fall  into  the  valleys  that  separate  them.  The  same 
effect  may  be  observed  with  any  object  floating  on  the 
water.  If,  however,  in  addition  to  being  in  a  state  of 
undulation  the  sea  is  really  in  motion  from  the  effects 
of  a  current,  or  from  any  other  cause,  the  floating 
object  will  of  course  be  carried  along  by  it — in  fact,  the 
two  movements  are  quite  independent  of  each  other, 
and  may  take  place  in  similar  or  contrary  directions. 
It  is  very  important  that  we  should  be  able  to  distin- 


WHAT  IS  LIGHT? 


81 


guish  at  an  early  period  the  exact  difference  between 
true  movement  and  mere  undulation ;  and  we  must  re- 
member that  although  the  waves  of  light  are  propa- 
gated at  the  rate  of  185,000  miles  a  second,  still  there 
is  no  transmission  of  any  material  substance  at  this 
marvellous  rate.  The  same  observation  applies  to 
sonorous  vibrations  transmitted  through  the  air. 

Thus  we  are  constrained  to  admit  peaceably  the  truth 
of  the  undulatory  hypothesis  as  compared  with  the  cor- 
puscular theory.  I  say  peaceably,  because  I  am  forcibly 
reminded  by  the  contrast  I  have  made  between  the  two 
theories  of  an  anecdote  related  of  one  of  the  greatest 
monsters  who  ever  walked  this  earth,  but  who  was  after- 
wards struck  down  in  the  midst  of  his  power  by  the 
hand  of  a  weak  girl.  I  allude  to  the  infamous  Marat, 
who  one  day  presented  himself  at  the  house  of  Dr. 
Charles,  a  celebrated  natural  philosopher,  of  the  time  of 
the  first  French  Republic,  in  order  to  advance  certain 
notions  of  his  own  against  the  optical  principles  that 
Newton  has  left  behind  in  his  Principia,  and  other 
works — also,  to  oppose  certain  theories  connected  with 
electrical  science.  Dr.  Charles,  who  did  not  approve  of 
Marat's  wild  notions,  undertook  to  convince  him  of  his 
errors.  But  instead  of  discussing  the  matter  p.  aceably, 
Marat  allowed  himself  to  be  carried  away  by  his  temper, 
which  was  naturally  very  violent.  Every  argument  ad- 
vanced by  his  antagonist  seemed  to  increase  his  rage, 
until  at  last  he  lost  all  control  over  himself,  drew  his 
small  sword,  and  rushed  upon  his  opponent.  The  doctor, 
who  was  unarmed,  had  to  exercise  all  his  powers  to  pre- 
vent himself  from  being  wounded,  and  being  much  more 
stoutly  built  than  Marat,  he  at  last  succeeded  in  throwing 
him  down,  and  wresting  his  sword  from  him,  which  he 
immediately  took  care  to  break.  Whether  it  was  the 
violence  of  the  fall,  the  shame  he  felt  at  being  doubly 
beaten,  or  the  effects  of  his  fit  of  passion,  does  not 


F 


82 


THE  WONDERS  OF  OPTICS. 


appear,  but  Marat  fainted.  Assistance  was  called,  and 
he  was  carried  home  to  his  house,  his  offence  against  all 
the  laws  of  propriety  being  forgiven  by  his  more  talented 
and  better-tempered  adversary. 

There  are  many  persons,  no  doubt,  whom  we  should 
astonish,  a^d  possibly  enrage,  by  asserting  positively 
that  we  could  cause  darkness  by  means  of  light,  that 
silence  could  be  produced  by  sound,  or  cold  by  heat. 
These  are  daring  paradoxes,  and  at  first  sight  appear 
almost  as  reasonable  as  that  of  Anaxagoras,  a  Greek 
philosopher,  who  asserted  that  snow  was  black.  But  as 
I  hope  that  most  of  my  readers  do  not  possess  the 
passionate  temper  of  the  French  tribune,  I  wilV  confide 
to  them  a  little  secret  that  will  make  these  paradoxes 
plain.  It  is  called  by  natural  philosophers  the  theory 
of  interference. 

The  experiments  connected  with  this  subject  are 
exceedingly  difficult  to  perform,  and  require  the  aid  of 
apparatus  far  beyond  the  reach  of  the  ordinary  student. 
It  is  a  case  where  theory  and  description  are  much  easier 
than  practice. 

If  a  ray  of  electric  light  is  thrown  upon  a  screen,  it  is 
possible  to  direct  another  ray  upon  the  same  spot  in 
such  a  manner  that  they  will  extinguish  each  other 
mutually.  The  reason  of  this  phenomenon  may  be 
understood,  if  we  remember  that  light  is  caused  by 
undulatory  movement,  and  that  by  opposing  two  series 
of  waves  to  each  other  in  such  a  manner  that  their 
vibrations  coming  in  contact  produce  rest,  we  can  easily 
see  how  the  waves  of  light  of  one  ray  may  be  stopped 
by  those  of  a  second. 

Going  back  to  our  illustration  of  the  eddies  on  a 
pool  of  water,  it  is  easy  to  prove  that  by  throwing 
a  second  stone  into  the  water  we  form  another  series 
of  undulations ;  which  are  mutually  destroyed  when 
they  encounter  each  other.    It  is  the  same  with  the 


WHAT  IS  LIGHT? 


peculiar  fluid  which,  existing  throughout  space,  is 
thrown  in  a  state  of  undulation  by  incandescent  bodies; 
by  opposing  one  set  of  waves  to  another  we  obtain  rest 
as  a  result. 

This  fact  was  first  observed  by  Grimaldi  in  1665,  and 
Dr.  Thomas  Young  was  the  first  to  offer  an  explanation. 
Fresnel  used  it  with  great  success  at  the  beginning  of 
the  century  to  demoLStrate  the  truth  of  the  undulatory 
theory,  by  showing  that  it  could  not  be  explained  by 
any  other. 


S4 


THE  WONDERS  OF  OPTrCS. 


CHAPTER  11. 

THE  SOLAR  SPECTRUM. 

The  white  light  that  the  glorious  orb  of  day  spreads 
over  the  face  of  nature  is  the  original  source  of  all  those 
brilliant  and  sombre  colours  with  which  the  works  of 
the  Creator  are  beautified.  To  the  rays  of  the  sun  we 
owe  not  only  the  whiteness  of  the  lily,  but  the  scarlet  of 
the  field  poppy,  the  modest  blue  of  the  timid  violet,  the 
splendour  of p  the  peacock's  plumage,  the  cool  green  of 
the  meadows,  and  the  purple  and  gold  of  the  distant 
mountains.  For,  as  we  have  hinted  before,  this  white 
light,  which  seems  of"  itself  so  destitute  of  colour,  is 
productive  of  every  hue  that  the  eye  of  man  is  capable 
of  appreciating. 

It  may  seem  that  I  am  bestowing  too  much  praise 
upon  our  own  sun  ;  but  if  you  are  surprised  that  I 
should  seek  to  exalt  this  brilliant  globe  of  ever-burning 
fire,  I  must  ask  you  to  recollect,  that  though  the  starry 
heavens  are  full  of  suns  as  vast  and  important  as  ours, 
and  possibly  affording  brilliant  colourless  light  to  worlds 
full  of  inhabitants,  there  are  others  that  give  forth  rays 
that  are  far  from  being  white.  Some  are  as  green  as 
emeralds,  others  are  as  blue  as  sapphires,  while  others 
give  out  a  warm  light  like  a  ruby  or  topaz.  The  worlds 
which  surround  these  can  only  receive  light  of  a  certain 
colour,  or  at  any  rate  they  are  restricted  to  a  few  shades 
and  hues.    Imagine  living  in  a  world  where  everything 


THE  S"LAR  SPECTRUM. 


85 


was  always  couleur  de  rose,  or  in  which  the  inhabitants 
were  continually  looking  blue !  A  residence  in  either 
of  them  for  a  short  time  would  undoubtedly  cause  us  to 
appreciate  the  relative  value  of  our  own  little  sun,  small 
as  it  is  in  comparison  with  some  of  the  mighty  orbs 
floating  about  in  space. 

The  fact  that  the  light  of  the  sun  is  the  source  of  all 
the  changing  hues  to  be  found  on  the  surface  of  the 
earth  season  after  season  was  first  discovered  by  Newton, 
and  his  experiments  are  easily  repeated  with  a  very  few 
and  inexpensive  appliances. 

A  small  round  hole  is  made  in  the  window-shutter  of 
a  room,  facing  the  sun,  and  the  pencil  of  light  proceed- 
ing from  it  is  allowed  to  fall  upon  the  surface  of  a  three- 
sided  prism,  held  in  a  horizontal  position,  and  placed 
at  a  distance  of  a  few  inches  from  the  aperture  (fig.  5, 
Frontispiece).  The  pencil  of  light  does  not  pass  through 
the  prism  as  if  it  were  a  plate  of  glass  with  parallel 
sides,  but  in  virtue  of  the  laws  of  refraction,  of  which 
we  have  already  spoken,  it  is  turned  out  of  its  natural 
course,  and  is  thrown  upon  the  wall  in  the  direction  in- 
dicated in  the  figure.  The  pencil  of  light  is  not  only 
turned  aside,  but  it  is  also  widened  out  into  a  band 
which  is  truly  painted  with  all  the  colours  of  the  rain- 
bow, every  tone  and  hue  being  of  the  most  marvellous 
brilliancy.  This  long  coloured  stripe,  which  constitutes 
one  of  the  most  beautiful  sights  that  the  science  of 
optics  can  afford  us,  is  known  to  scientific  men  by  the 
name  of  the  solar  spectrum. 

Bjfore  going  into  the  causes  that  produce  these 
colours,  let  us  first  examine  their  number  and  position. 
Beginning  at  the  top,  we  shall  find  that  they  run  in  the 
following  order  : — Violet,  indigo,  blue,  green,  yellow, 
orange,  red.  The  red  being  lowest  is  called  the  least 
refrangible  of  them  all ;  or,  in  other  words,  in  passing 
through  the  prism  it  was  bent  less  out  of  its  course  than 


86 


THE  WONDERS  OF  OPTICS. 


its  companions.  Violet,  being  at  the  top,  is  of  course 
the  most  refrangible.  The  cause  of  the  separation  of 
the  colours  of  white  light  is  consequently  only  the  effect 
of  their  individual  character.  They  were,  so  to  speak, 
so  many  streams  flowing  together  until  an  unexpected 
deviation  in  their  course  caused  them  to  separate.  This 
change  in  the  direction  of  their  flow  brought  out  their 
personal  individuality,  and  they  at  once  became  com- 
pletely disunited. 

Every  single  tint  in  the  prismatic  spectrum  is  simple, 
and  cannot  be  decomposed.  This  may  be  shown  by 
passing  any  of  them  through  another  prism,  when  it 
will  be  found  that  no  change  will  take  place  in  the 
colour  or  size  of  the  pencil.  Hence  those  worlds  already 
spoken  of,  whose  light  of  day  is  red,  blue,  or  green, 
never  see  any  colours  but  these.  (Fig.  6,  Frontispiece). 

It  is  just  as  easy  to  reunite  the  colours  into  which 
white  light  is  decomposed,  by  applying  a  second  prism 
in  a  reversed  position  to  the  pencil  of  coloured  light,  as 
it  is  to  separate  them  in  the  first  instance.  The  method 
of  accomplishing  this  is  shown  in  fig.  7,  Frontispiece. 


Fig.  8.— The  Recom position  of  Light. 

Another  experiment  in  the  same  direction  consists  in 


THE  SOLAR  SPECTRUM.  87 


reuniting  the  colours  by  causing  them  to  pais  through 
a  double  convex  lens,  behind  which  is  placed  a  screen 
of  ground  glass,  or  a  card  (fig.  8).  By  advancing  and 
withdrawing  this  screen  we  can  easily  find  the  exact 
spot  where  the  rays  reunite,  and  form  a  dazzling  spot 
of  white  light.  This  point  is  called  the  focus,  from  a 
Latin  word,  signifying  "  fire-place,,,  a  term  which  will 
put  the  student  in  mind  of  the  frequently  repeated  ex- 
periment of  burning  a  piece  of  paper  with  an  ordinary 
magnifying-glass. 

Instead  of  using  a  lens,  you  can,  if  you  please,  em- 
ploy a  concave  mirror,  using  the  ground  glass  or  card- 


Fig.  9. — Recomposition  ot  Light  by  means  of  a  Concave  Mirror. 


board  screen,  as  before.  The  colours  reflected  by  the 
mirror  unite  at  its  focus,  and  produce  a  brilliant  wrhite 
spot  in  just  as  conclusive  a  manner  as  in  the  other  ex- 
periment. 

A  fourth  experiment,  which  is  somewhat  more  diffi- 
cult for  the  student  to  accomplish,  consists  in  causing 
every  one  of  the  seven  different  colours  to  be  reflected 
from  a  separate  mirror. 

The  mirrors  in  this  case  are  concave,  and  are  so 
mounted  as  to  be  capable  of  being  moved  in  any  direc- 
tion. By  directing  each  of  the  seven  rays,  one  by  one, 
upon  the  same  point,  you  may  observe  the  gradual  de- 


88 


THE  WONDERS  OF  OPTICS. 


composition  of  the  coloured  light.  The  effect  obtained 
by  adding  the  last  colour  to  the  mixture  is  quite  magi- 
cal, the  white  circle  being  produced  from  two  brilliantly- 
coloured  spots. 


Fig.  10.— Recomposition  of  Light  by  means  of  a  number  of  Mirrors. 


A  fifth  experiment,  first  devised  by  Newton,  is  also 
within  the  reach  of  the  student.  On  a  disc  of  cardboard 
the  centre  and  border  of  which  have  been  previously 
painted  black,  are  pasted  seven  strips  of  paper,  painted 
as  nearly  as  possible  of  the  same  colour  as  the  compo- 
nents of  the  spectrum — or  if  the  student  is  anything 
of  an  artist  he  may  paint  the  disc  in  imitation  of  the 
spectrum,  carefully  shading  off  the  tints  into  each  other. 
If  the  disc  be  now  rapidly  rotated  the  colours  will  dis- 
appear, and  a  greyish  hue  will  be  seen,  which  will  ap- 
proach more  "closely  to  white>  the  nearer  the  colours  on 
the  disc  are  to  those  of  the  spectrum,  This  experiment 
is  not  precisely  the  same  in  principle  as  the  preceding 
ones,  for  it  is  evident  that  the  colours  on  the  disc  do 
not  mix,  but  only  the  impressions  they  form  upon  the 
retina.  We  have  already  said  that  such  impressions 
remain  on  the  eye  for  one-tenth  of  a  second  or  there- 
abouts ;  the  disc  must  therefore  revolve  at  least  ten 
times  a  second,  or  the  effect  will  not  be  perceived. 

From  these  experiments  it  follows  that  the  colours 


THE  SOLAR  SPECTRUM. 


89 


with  which  all  natural  substances  are  clothed,  ought  not 
to  be  looked  upon  as  belonging  to  them  absolutely,  but 
only  as  a  property  dependent  on  the  reflection  and 
absorption  of  light  from  their  surfaces.    The  leaves  of 


Fig.  11. — Newton's  Disc. 


plants,  for  instance,  must  not  be  regarded  as  being 
really  green  in  themselves,  but  as  being  capable  of  ab- 
sorbing certain  portions  of  light,  and  reflecting  others. 
Grown  in  the  dark,  the  green  substance  contained  in  the 
plant  and  its  leaves  becomes  white,  and  no  longer  pos- 
sesses the  property  of  absorbing  red  light,  and  reflect- 
ing green.  A  green  leaf  placed  in  red  light  becomes  al- 
most black,  from  its  power  of  absorbing  light  of  that 
colour;  in  the  blue  it  reflects  a  much  greater  proportion 
of  the  coloured  ray.  A  very  striking  experiment  may 
be  performed  with  a  substance  known  to  chemists  as  the 


90 


THE  WONDERS  OF  OPTICS. 


iodide  of  mercury.  If  a  little  of  this  salt,  which  is  of  a 
brilliant  red,  be  placed  in  a  watch-glass,  and  heated  over 
a  spirit-lamp,  it  will  gradually  sublime,  and  a  card  held 
over  it  will  be  covered  with  a  number  of  light  yellow 
crystals.  In  this  case  no  change  of  composition  has 
taken  place,  but  simply  a  change  in  the  power  the  salt 
possesses  of  reflecting  some  rays  and  absorbing  others. 
By  simply  scratching  the  surface  of  the  card  with  a 
pointed  piece  of  wood,  the  yellow  crystals  become  trans- 
formed once  more  into  the  red  variety ;  not  only  this, 
the  transformation  gradually  spreads,  like  a  red  cloud, 
over  the  whole  of  the  deposit.  There  are  some  other 
salts  known  to  chemists  which  possess  the  property  of 
dichroism,  or  double  colour.  The  double  cyanide  of 
platinum  and  barium,  for  instance,  appears  violet  when 
viewed  in  one  direction,  and  yellow  in  another.  Change 
of  temperature  is  often  sufficient  to  change  the  colour  of 
bodies — white  oxide  of  zinc,  for  example,  becomes  bright 
yellow  when  heated.  Such  instances  might  be  supplied 
ad  infinitum,  but  enough  has  been  said  to  prove  that 
colour,  after  all,  is  only  an  appearance,  and  not  an  es- 
sential property  of  bodies. 

We  have  already  spoken  of  complementary  colours, 
or  those  which  it  is  necessary  to  add  together  in  order  to 
produce  wThite  light.  Blue,  for  instance,  is  complemen- 
tary to  orange,  red  to  green,  violet  tp  yellow,  and  vice 
versa.  But  it  is  not  by  the  aid  of  the  palette  that  this 
can  be  proved,  for  in  the  case  of  coloured  pigments  the 
arrangement  of  their  atoms  interferes  in  some  way  with 
the  success  of  the  experiment,  and  it  is  only  by  means 
of  the  colours  of  the  spectrum  that  such  recompositions 
can  be  effected. 

Although  most  philosophers  consider  that  there  are 
seven  colours  in  the  spectrum,  there  are  others  who  do 
not  admit  it,  but  assert  that  there  are  really  only  three, 
red,  yellow  and  blue — which  by  the  superposition  of 


THE  SOLAR  SPECTRUM. 


91 


their  edges  produce  the  intermediate  hues  of  green  and 
orange.  Perhaps  it  would  be  nearer  to  the  truth  to  say 
that  the  spectrum  is  composed  of  an  infinite  number  of 
colours  of  different  hues. 

We  have  already  stated  that  every  one  of  these 
colours  is  indecomposable,  and  that  there  are  certain 
worlds  illuminated  by  a  single  colour  only,  instead  of 
possessing  the  infinite  number  of  tints  enjoyed  by  the 
inhabitants  of  the  solar  system.  An  idea  of  this  effec.t 
can  easily  be  gained  in  a  very  simple  but  surprising 
manner  by  inserting  panes  of  glass  of  different  colours 
in  the  hole  of  the  shutter  of  a  dark  room.  If  the  light 
is  yellow,  you  will  find  that  all  those  objects  that  are 
capable  of  reflecting  yellow  light  are  coloured  by  it, 
while  those  which  are  bright  red  or  blue  become  almost 
black  by  absorbing  the  only  light  present.  If  we  could 
procure  an  object  which  was  perfectly  complementary 
in  colour  to  the  yellow  glass,  it  would  appear  perfectly 
black.  The  same  experiment  may  be  repeated  with 
the  other  colours.  After  remaining  in  this  coloured 
light  for  some  time,  if  you  suddenly  pass  out  into  day- 
light the  complementary  colour  will  tinge  everything 
around  you. 

Instead  of  using  a  room  into  which  coloured  light 
only  is  admitted,  lamps  burning  with  a  coloured  flame 
may  be  employed.  Brewster  mentions  the  following 
experiment,  which  is  a  very  striking  one  : — Fill  a  spirit- 
lamp  with  alcohol  in  which  has  been  dissolved  as  much 
common  salt  as  the  spirit  will  take  up  ;  on  being  lit  it 
will  be  found  to  burn  with  a  livid  yellow  flame.  A 
voom  lighted  entirely  with  one  or  two  lamps  of  this 
kind  will  form  a  laboratory  for  some  very  singular  ex- 
periments. It  should,  if  possible,  be  hung  with  pictures 
in  water  and  oil  colours,  and  the  persons  present  ought 
to  wear  nothing  but  the  brightest  colours,  and  the  table 
be  ornamented  with  the  gayest  of  flowers.     The  room 


92 


THE  WONDERS  OF  OPTICS. 


being  first  lighted  with  ordinary  daylight,  the  lamps 
above  mentioned  should  be  brought  in,  and  the  day- 
light carefully  excluded,  when  an  astonishing  metamor- 
phosis will  take  place.  The  spectators  will  be  hardly 
able  to  recognise  each  other  ;  the  furniture  of  the  room, 
and  every  other  object  contained  in  it,  will  reflect  but  a 
single  colour.  The  flowers  will  lose  their  brilliant 
tints,  the  paintings  will  appear  as  if  they  were  drawn 
in  Indian  ink.  The  brightest  purple,  the  purest  lilac, 
the  richest  blue,  the  liveliest  green,  will  be  converted 
into  a  monotonous  yellow.  The  same  change  will  take 
place  in  the  countenances  of  those  present;  a  livid 
paleness  will  spread  over  their  faces,  whether  young  or 
old,  and  those  who  are  naturally  of  an  olive  complexion 
will  hardly  appear  changed  at  all.  Every  one  will 
laugh  at  the  appearance  of  his  neighbour's  face,  without 
thinking  that  he  is  just  as  great  a  subject  of  laughter  to 
them.  If,  in  the  midst  of  the  amusement  caused  by  this 
experiment,  the  light  of  day  is  admitted  at  one  end  of 
the  room,  the  other  end  being  still  lighted  with  the 
salt-lamp,  i  very  one  will  appear  to  be  half-illuminated 
with  the  livid  colour  which  has  caused  so  much  surprise, 
the  other  portion  of  their  figure  and  clothes  being  of 
the  natural  hue.  One  cheek,  for  instance,  will  appear 
animated  with  its  usual  brilliancy,  while  the  other  will 
be  that  of  a  corpse ;  one  side  of  a  lady's  dress  will  be 
brilliant  blue  or  green,  as  the  case  may  be,  the  other  a 
colour  that  it  would  puzzle  an  artist  to  give  a  name  to. 
The  experiment  may  be  varied  by  admitting  the  white 
light  through  several  small  holes  in  the  shutter  of  the 
room,  every  luminous  spot  painting  the  place  where  it 
falls  in  its  natural  colours,  and  the  yellow  spectators 
will  become  spotted  with  the  most  singular  tints  and 
hues.  If  a  magic-lantern  is  used  to  throw  on  the  walls 
of  the  room  and  the  clothes  of  the  company  any  lumi- 
nous figures,  such  as  those  of  flowers  or  animals,  they 


THE  SOLAR  SPECTRUMc 


93 


will  be  coloured  with  these  figures  in  the  tint  of  the 
wall  or  fabric  upon  which  they  fall,  yellowish  colours 
of  course  escaping  the  transformation.  If  nitrate  of 
strontia  be  substituted  for  the  salt,  a  crimson  tint  will 
be  spread  over  everything.  In  fact,  a  lamp  prepared 
in  this  wav  will  form  a  source  of  endless  amusement. 
It  is  not  necessary  to  use  alcohol  for  the  purpose; 
wood-spirit  or  methylated  alcohol  will  serve  the  purpose 
equally  well.  If  a  lamp  is  not  to  be  had,  a  few  pieces 
of  cotton-wool,  tied  on  wires  and  dipped  in  the  salted 
spirit,  will  do  almost  as  well. 


94 


THE  WONDERS  OF  OPTICS. 


CHAPTER  III. 

OTHER  CAUSES  OF  COLOUR, 

The  colours  of  the  spectrum  are  to  the  sense  of  sight 
what  the  tones  of  the  gamut  are  to  the  sense  of  hear- 
ing. On  the  one  hand,  the  differences  in  the  lengths 
of  the  sonorous  waves  constitute  the  variety  of  note  per- 
ceptible by  the  ear ;  on  the  other,  the  differences  in  the 
lengths  of  the  luminous  waves  constitute  the  variety  of 
colour  perceptible  by  the  eye.  By  and  by,  we  shall 
learn  both  the  length  and  rapidity  of  these  vibrations, 
but  it  will  be  as  well  first  to  describe  the  experi- 
ments made  in  this  direction  by  the  immortal  Newton 
himself. 

Every  one  has,  doubtless,  at  one  period  of  his  life, 
amused  himself  with  blowing  soap-bubbles  by  means  of 
a  tobacco-pipe  and  a  little  lather — a  sufficiently  childish 
amusement,  you  will  possibly  say,  but  one  narrowly 
connected  with  the  most  intricate  secrets  of  the  science 
of  optics.  These  little  globes,  so  fragile  that  they  dis- 
appear in  a  breath,  hardly  seem  worthy  of  the  attention 
of  a  thinker,  and  still  less  the  examination  of  a  philoso- 
pher ;  but  it  is  nevertheless  true  that  Newton  made  ex- 
periments on  the  colours  shown  on  the  surface  of  these 
apparently  insignificant  objects  which  ended  in  the 
most  brilliant  discoveries,  just  as  on  seeing  an  apple  fall 
he  began  a  train  of  thought  which  only  terminated  in 
the  enunciation  of  the  hypothesis  of  the  earth's  power 
of  gravity. 


OTHER  CAUSES  OF  COLOUR. 


95 


All  transparent  substances,  whether  liquid,  solid,  or 
gaseous,  become  coloured  with  the  most  brilliant  hues 
as  soon  as  they  are  reduced  to  plates  of  extreme  thin- 
ness. In  the  soap-bubble  it  is  the  oleaginous  particles 
floating  on  the  surface  which  thus  become  coloured,  but 
Newton  showed  that  thin  plates  of  air  were  similarly 
capable  of  showing  colour,  and  that  the  thinner  the 
plates  were  the  more  brilliant  were  the  tints.  We  may 
see  this  in  the  soap-bubble,  which  becomes  more  beau- 
tiful as  it  gets  larger  and  thinner.  By  placing  a  con- 
vex lens  of  large  size  on  a  flat  plate  of  glass,  Newton 
observed  that  rings  of  different  colours  were  formed 
round  the  spot  where  the  two  pieces  of  glass  touched. 


Fig.  12. — Newton's  Rings. 

By  measuring  the  convexity  of  the  lens  and  the  di- 
ameter of  the  various  rings,  Newton  was  enabled  to  tell  to 
a  minute  fraction  the  exact  thickness  of  the  plate  of 
air  corresponding  to  the  different  colours.  The  glasses 
being  placed  in  position,  a  ray  of  a  particular  colour — 
red,  for  instance — was  thrown  upon  the  surface.  The 
result  was  a  black  spot  at  the  point  where  the  two  sur- 
faces touched,  and  surrounding  it  at  various  distances 
were  several  rings  alternately  red  and  black.  Calcu- 
lating the  thickness  of  the  plates  of  air  at  the  part 
where  the  dark  rings  made  their  appearance,  Newton 
found  that  their  dimensions  were  in  the  proportion  of 
the  even  numbers  two,  four,  six,  eight,  &c. ;  while  the 
red  rings  showed  figures  corresponding  to  the  odd  num- 
bers. Although  trammelled  by  the  corpuscular  theory, 
Newton's  deductions  from  these  experiments  show  that 


96 


THE  WONDERS  OF  OPTICS. 


they  can  only  be  accounted  for  by  the  undulatory  hypo- 
thesis. Thus  the  thickness  of  the  plate  of  air  at  the  first 
red  ring  is  that  of  the  red  wave,  the  thickness  at  the 
second  that  of  two  red  waves,  and  so  on ;  so  that  in  or- 
der to  arrive  at  the  thickness  of  the  red  wave  we  need 
only  measure  the  distance  between  the  portions  of  tK 
glasses  where  the  first  red  ring  occurs. 

This  experiment  was  applied  to  the  measurement  of 
all  the  waves.  Whenever  they  were  reflected  on  the 
glasses  a  parallel  series  of  rings  was  formed,  but  it  was 
found  that  the  first  ring  was  more  or  less  distant  from 
the  central  spot,  according  to  the  colour  used.  The  red 
ring  was  the  largest ;  the  orange,  yellow,  green,  blue, 
indigo,  and  violet,  following  in  the  same  sequence  as  in 
the  spectrum.  The  word  "  thickness"  seems  hardly  fit 
to  apply  to  dimensions  arrived  at  by  Newton  in  his  ex- 
periments, so  infinitely  small  do  they  appear  to  be,  yet 
their  correctness  has  never  been  impugned,  although 
the  experiments  have  been  repeated  by  the  philosophers 
of  all  countries.  The  waves  of  red  light  are  so  small 
that  40,000  of  them  go  to  an  inch,  and  those  of  violet 
light  situated  at  the  other  end  of  the  spectrum  are  still 
smaller,  measuring  only  the  60,000th  part  of  an  inch. 

The  waves  of  the  other  colours  are  between  these  two, 
while  the  wave  of  white  light,  which  is  a  mixture  of 
them  all,  is  just  half-way  between  the  two. 

Thus  was  the  physical  cause  of  the  various  hues  of 
colour  discovered  by  this  great  man,  revealing  as  it  does 
the  singular  and  mysterious  analogy  between  sound  and 
light.  The  rays  of  light,  like  the  waves  of  sound,  pro- 
duce a  different  effect,  according  to  their  length,  by 
causing  quicker  or  slower  pulsations  in  the  nerves  of 
sight,  just  as  musical  sounds  vibrate  upon  the  drum  of 
the  ear  with  different  velocities. 

This  is  not  all,  for  the  relationship  between  sound  and 
light  does  not  cease  here:  we  have  as  yet  only  spoken 
of  the  size  of  the  undulations,  and  have  only  shown 


OTHEK  CAUSES  OF  COLOUR. 


9T 


how  their  dimensions  are  connected  with  the  sensation 
of  colour;  but  there  are  other  things  to  be  considered, 
for  on  investigation  we  find  that  not  only  do  the  dif- 
ferent coloured  waves  vary  in  the  length  of  their  undula- 
tions, but  also  in  the  number  that  take  place  in  a  given 
time. 

The  perception  of  sound  is  produced  by  the  action  of 
the  drum  of  the  ear,  which  vibrates  sympathetically 
with  the  pulsations  of  the  air  that  have  been  originated 
by  the  vibrations  of  the  sounding  body  ;  and  the  per- 
ception of  light  is  produced  in  a  similar  manner  by  the 
vibrations  originating  in  a  luminous  body,  and  propa- 
gating themselves  through  the  luminous  ether  until 
they  reach  the  nerves  of  sight.  The  number  of  these 
pulsations  taking  place  in  the  eye  has  been  accurately 
determined  in  the  following  manner.  Let  us  suppose 
that  we  are  looking  at  a  coloured  object — let  us  say,  a 
red  railway  signal-lamp  ;  from  the  lamp  to  our  eye  there 
flows  a  continuous  line  of  luminous  undulations;  these 
undulations  enter  the  eye  and  become  depicted  on  the 
retina.  For  every  wave  that  passes  through  the  pupil, 
there  is  a  separate  and  corresponding  vibration  of  the 
optic  nerve,  and  the  number  of  these  vibrations  that 
take  place  in  the  course  of  a  second  can  be  easily  cal- 
culated if  we  know  the  velocity  of  light  and  the  breadth 
of  the  waves.  We  have  before  found  that  light  travels 
at  the  rate  of  185,000  miles  per  second;  it  therefore 
follows,  that  a  series  of  undulations  185,000  miles  long 
pass  through  the  pupil  every  second ;  consequently  the 
number  of  vibrations  per  second  is  arrived  at  by  cal- 
culating how  many  waves  measuring  the  40,000th  of 
an  inch — that  being  the  length  of  a  wave  of  red  light — 
are  contained  in  185,000  miles.  The  following  table, 
showing  the  number  of  waves  passing  into  the  eye 
per  second  for  the  different  colours,  will  interest  the 
student  :— 

6 


98 


THE  WONDERS  OF  OPTICS. 


Extreme  red 
Red  . 
Orange 
Yellow 
Green 
Blue 
Indigo 
Violet 


458,000,000,000,000  waves  per  second. 
477,000,000,000,000 
506,000,000,000,000 

535,000,000,000,000  " 

577,000,000,000,000  " 

622,000,000,000,000  « 

658,000,000,000,000  " 

699,000,000,000,000  " 


Extreme  violet    .  727,000,000,000,000 

Whatever  theory  we  may  adopt  to  explain  the  phe- 
nomena of  light,  we  arrive  at  conclusions  that  strike 
the  mind  with  astonishment  and  admiration.  Accord- 
ing to  the  corpuscular  hypothesis,  it  was  supposed  that 
the  molecules  of  light  were  endowed  with  the  power  of 
attraction  and  repulsion,  that  they  possessed  poles  and 
centres  of  gravity  like  the  earth,  and  that  they  had 
other  physical  properties  that  could  only  be  given  to 
ponderable  matter.  Starting  with  these  notions,  it  is 
difficult  to  divest  oneself  of  the  idea  of  sensible  size,  or 
to  induce  the  mind  to  conceive  particles  so  extremely 
small  as  those  of  light  would  necessarily  be  if  the  theory 
of  emission  were  accepted.  If  a  particle  of  light  weighed 
a  grain,  it  would  produce  by  means  of  its  enormous  ve- 
locity the  effects  of  a  cannon-ball  weighing  120  lbs., 
travelling  at  the  rate  of  300  yards  per  second.  How 
infinitely  small  would  be  these  particles,  seeing  that  the 
most  delicate  optical  instruments  are  submitted  to  their 
action  for  years  without  being  injured  ! 

If  we  are  astonished  at  the  extreme  smallness  and 
prodigious  rapidity  of  the  luminous  molecules  whose 
existence  is  necessitated  by  the  corpuscular  theory,  the 
numerical  results  of  the  undulatory  hypothesis  are  not 
less  surprising.  The  extreme  smallness  of  the  distance 
between  the  waves,  and  the  inconceivable  quickness  of 
their  undulations,  although  both  are  easily  calculated, 
must  raise  in  the  mind  of  the  student  feelings  of  the 
utmost  wonder  and  admiration. 


OTHER  CAUSES  OF  COLOUR. 


99 


Colour,  then,  simply  results  from  the  difference  in 
the  rate  of  vibration  of  the  rays,  as  Professor  Tyndall 
observes  in  his  lectures  on  the  "  Analogy  between  Sight 
and  Sound,"  the  impression  of  red  being  produced  by 
waves  that  undulate  a  third  less  rapidly  than  those 
which  produce  the  sensation  of  violet. 


100 


THE  WONDERS  OF  OPTICS- 


CHAPTER  IV. 

LUMINOUS,   CALORIFIC,  CHEMICAL,  AND  MAGNETIC  PRO- 
PERTIES OF  THE  SPECTRUM. 

The  solar  spectrum  may  be  compared  to  a  battle-field 
with  an  army  drawn  up  upon  it  ready  for  action.  In 
the  centre  we  find  the  luminous  rays,  on  one  side  the 
light  troops  which  produce  chemical  effect,  and  on  the 
other  the  heating  rays,  which  may  be  compared  to  squad- 
rons of  heavy  cavalry.  Close  by  the  light  brigade  are 
the  magnetic  rays,  w;hich  are  a  corps  of  skirmishers, 
sometimes  appearing,  and  at  others  hiding  themselves 
from  view  in  a  very  mysterious  manner. 

But  to  drop  metaphor,  we  shall  find  on  examination 
of  the  spectrum  that  the  three  great  forces — heat,  light, 
and  chemical  effect — are  regularly  distributed  over  three 
different  portions  of  this  wonderful  band  of  colour. 

Before  Fraunhofer  the  intensity  of  the  light  of  dif- 
ferent parts  of  the  spectrum  remained  undetermined 
with  any  degree  of  accuracy ;  but  this  philosopher,  by 
the  use  of  a  very  delicate  photometer,  obtained  the  re- 
sults given  below. 

The  maximum  of  luminous  effect  is  situated  just  at 
the  junction  of  the  yellow  and  orange.  Taking  this 
spot  as  its  starting-point,  it  gradually  decreases  on  each 
side  until  it  ceases  altogether  at  the  extreme  red  and 
violet. 

With  respect  to  the  calorific  portion  of  the  spectrum 


PROPERTIES  OF  THE  SPECTRUM.  101 


it  was  for  a  long  time  supposed  that  the  heat-giving 
properties  of  any  part  were  in  direct  proportion  to  the 
amount  of  its  luminous  effect ;  but  Sir  John  Herschel 
proved  by  a  long  series  of  experiments  that  the  heat  of 
the  spectrum  gradually  increased  from  the  extreme 
violet  to  the  extreme  red,  and  that  passing  this  point  it 
still  further  increased  until  it  attained  its  maximum  at 
a  point  where  not  a  single  ray  of  light  existed.  From 
these  grand  experiments  he  adduced  the  important  con- 
clusion, that  in  solar  light  there  existed  invisible  rays, 
which  produced  heat,  and  which  possessed  even  a  less 
degree  of  refrangibility  than  the  extreme  red  rays.  Sir 
John  Herschel  then  tried,  but  unsuccessfully,  to  deter- 
mine the  exact  refrangibility  of  the  invisible  heat 
rays. 

Sir  Henry  Englefield  compared  these  results,  and 
obtained  the  following  figures  : — 

Blue   .    .  . 
Green 

Yellow  .  . 
Red  .  .  . 
Beyond  the  red 

Berard  obtained  similar  results,  but  he  at  first  found 
that  the  maximum  of  heat  was  just  at  the  end  of  the 
extreme  red,  and  that  beyond  it  the  air  was  only  about 
one-fifth  warmer  than  the  ordinary  temperature.  Sir 
John  Herschel  attributed  these  discordant  results  to 
Berard  having  used  a  thermometer  with  too  large  a 
bulb ;  he  accordingly  repeated  his  experiments  with 
other  instruments  with  long  narrow  bulbs,  and  arrived 
at  similar  results  to  those  obtained  by  the  English 
philosopher. 

We  will  now  pass  on  to  the  physical  properties  of  the 
other  end  of  the  spectrum.  Towards  the  end  of  the  last 
century,  Scheele,  a  Swedish  philosopher,  remarked  that 


56  deg.  Fahr. 

58 

62 

72 

79  " 


102 


THE  WONDERS  OF  OPTICS. 


chloride  of  silver  was  blackened  more  quickly  by  the 
violet  portion  of  the  spectrum  than  by  any  other.  In 
1801,  Ritter  of  Genoa,  in  repeating  certain  experiments 
made  by  Herschel,  found  that  a  much  stronger  black- 
ening effect  was  produced  at  a  point  beyond  the  violet, 
and  that  the  discoloration  was  produced  with  less  in- 
tensity by  the  violet  and  still  less  so  by  the  blue,  the 
change  gradually  decreasing  till  the  red  ray  was  reached. 
He  also  found  that  when  slightly  blackened  chloride  of 
silver  was  exposed  to  the  effects  of  the  red  rays,  or 
even  in  the  space  beyond,  its  colour  was  restored  to  it. 
From  these  facts  he  drew  the  conclusion  that  in  the 
solar  spectrum  there  existed  two  kinds  of  rays,  one  at 
the  red  extremity,  which  favoured  oxygenation  ;  the 
other,  at  the  blue  end,  which  possessed  the  contrary 
properties.  He  also  found  that  when  phosphorus  was 
placed  in  the  invisible  rays  beyond  the  red,  it  gave  off 
fumes  of  oxide,  which  were  immediately  extinguished 
when  it  was  transferred  to  the  other  end. 

On  repeating  the  experiment  with  chloride  of  silver, 
Lubeck  found  that  the  tint  varied  according  to  the 
colour  in  which  it  was  placed.  Beyond  or  in  the  violet 
ray  it  became  brownish  red,  in  the  blue  it  became 
bluish  or  bluish  grey,  in  the  yellow  it  remained  white, 
or  became  slightly  yellow  and  reddish  in  or  beyond  the 
red  ray.  When  he  used  prisms  of  flint  glass,  the  chlo- 
ride of  silver  was  discoloured  beyond  the  visible  limits 
of  the  spectrum. 

Without  being  aware  of  Hitter's  experiments,  Dr. 
Wollaston  obtained  the  same  results  by  acting  on 
chloride  of  silver  with  violet  light.  In  continuing  his 
researches  he  discovered  that  gum  guaiacum  was  also 
influenced  by  the  chemical  rays  of  light. 

The  magnetic  influence  supposed  to  be  exerted  by  the 
solar  rays  still  remains  without  positive  proof,  although 
numbers  of  philosophers  have  experimented  in  this 


PROPERTIES  OF  THE  SPECTRUM. 


103 


direction.  More  than  fifty  years  ago  Dr.  Morichini 
announced  that  the  violet  rays  of  the  solar  spectrum 
possessed  the  property  of  magnetizing  steel  needles 
that  were  previously  free  from  magnetism.  He  pro- 
duced this  effect  by  concentrating  the  violet  rays  upon 
one-half  of  each  needle  with  a  convex  lens,  taking  care 
to  keep  the  other  half  concealed  beneath  a  screen. 
After  having  continued  this  experiment  for  more  than 
an  hour,  the  nee  lies  were  found  to  be  quite  magnetic. 

Dr.  Somerville  tested  Morichini's  experiments  by 
covering  one-half  of  an  unmagnetized  needle  an  inch 
long  with  a  piece  of  paper,  and  exposing  the  uncovered 
half  to  the  violet  rays  of  the  spectrum,  and  found  that 
the  needle  became  magnetic  in  the  course  of  a  couple 
of  hours,  the  exposed  end  being  the  north  pole.  The 
indigo  rays  produced  almost  the  same  effect,  but  the 
blue  and  green  rays  were  much  less  powerful.  When 
the  needle  was  exposed  to  the  yellow,  orange,  red,  and 
invisible  rays  beyond  the  red,  no  magnetic  effect  was 
produced,  although  the  experiment  was  continued  for 
three  days.  Pieces  of  chronometer  and  watch  springs 
were  submitted  to  the  same  influences  with  a  similar 
result ;  but  when  the  violet  rays  were  concentrated  upon 
the  needles  and  pieces  of  spring  with  a  lens,  the  time 
necessary  for  magnetizing  them  was  greatly  reduced. 

Baumgartner  of  Vienna  and  Christie  of  Woolwich 
also  repeated  these  experiments.  The  latter  philosopher 
found  that  when  a  needle  of  magnetized  steel,  copper, 
or  even  glass,  vibrated  by  force  of  torsion  in  the  rays 
of  the  sun,  the  arc  of  vibration  diminished  much  more 
quickly  than  when  the  experiment  was  conducted  in  the 
shade.  The  sun's  rays  appeared  to  have  the  greatest 
effect  upon  the  magnetized  needle.  From  these  results 
Christie  concluded  that  the  solar  rays  were  capable  of 
exerting  a  certain  amount  of  magnetic  influence. 

These  experiments  were  afterwards  fully  confirmed 


104 


THE  WONDERS  OF  OPTICS. 


by  those  of  Barlocci  and  Zantedeschi.  The  former 
found  that  a  natural  magnet  which  was  capable  of  sup- 
porting a  pound  weight,  had  its  power  almost  doubled 
by  exposure  to  strong  sunlight  for  four-and-twenty 
hours.  Zantedeschi  exposed  a  magnet  which  would 
carry  fifteen  ounces  to  the  sun  for  three  days,  and  in- 
creased its  power  two  and  a  half  times.  These  experi- 
ments seem  almost  to  decide  the  fact  of  the  power  of 
white  and  violet  light  to  induce  magnetic  force;  but  a 
series  of  researches  by  a  philosopher  who  without  doubt 
is  greater  than  any  of  those  already  mentioned,  seems 
to  throw  some  doubt  on  the  facts  we  have  related  above. 

Before  concluding,  we  must  add  a  few  more  facts  re- 
lating to  the  existence  of  invisible  rays  at  both  ends  of 
the  spectrum.  "  The  visible  portion  of  the  spectrum," 
says  Dr.  Tyndall,  in  one  of  his  Royal  Institution  lec- 
tures, "  simply  marks  an  interval  of  radiant  action,  the 
rays  existing  in  which  bear  such  a  relation  to  our  visual 
organs,  as  to  be  capable  of  exciting  in  them  the  sensa- 
tion of  light.  Beyond  this  interval,  in  both  directions, 
right  and  left,  the  radiant  action  continues  to  exercise 
itself,  but  the  rays  emitted  are  dark,  in  consequence  of 
their  exerting  no  influence  on  our  eye.  Those  that  ex- 
ist beyond  the  red  ray  are  capable  of  producing  heat, 
while  those  that  are  beyond  the  violet  excite  chemical 
action.  These  invisible  violet  rays  can  be  actually 
made  perceptible  to  the  eye,  or,  in  other  words,  the  un- 
dulations or  waves  proceeding  from  this  end  of  the 
spectrum  can  be  made  to  strike  against  certain  sub- 
stances and  induce  luminous  vibrations,  so  as  to  connect 
the  dark  space  beyond  the  violet  with  a  brilliantly  illu- 
minated band.  I  have  here  a  substance  capable  of 
effecting  this  change.  The  lower  half  of  this  sheet  of 
paper  has  been  moistened  with  a  solution  of  sulphate 
of  quinine,  the  other  half  being  left  in  its  ordinary 
condition.  I  will  now  hold  the  paper  in  such  a  manner 
that  the  line  that  separates  the  prepared  half  from  the 


PROPERTIES  OF  THE  SPECTRUM. 


105 


other  shall  cut  the  spectrum  in  two  halves  horizontally. 
The  upper  half  will  remain  unaltered  and  may  be 
readily  compared  with  the  lower  half,  upon  which  you 
will  see  the  spectrum  prolonged  beyond  its  ordinary 
limits.  The  effect  produced  is  the  addition  of  a  splendid 
band  of  fluorescent  light,  which  extends  over  a  space  of 
several  inches,  which  but  an  instant  before  was  a  dark 
"mass.  I  withdraw  the  prepared  paper,  and  the  light 
disappears  ;  I  replace  it,  and  the  light  shines  forth  once 
more  ;  showing  us  in  the  most  brilliant  way  that  the 
visible  limits  of  the  ordinary  spectrum  are  not  the  lim- 
its of  radiant  action. 

"  I  plunge  a  pencil  into  the  solution  of  sulphate  of 
quinine,  and  I  pass  it  over  the  paper.  You  see  that 
wherever  the  solution  falls,  the  light  bursts  forth.  The 
existence  of  these  rays  has  been  known  for  a  long  time. 
Young  was  familiar  with  them,  and  subjected  them  to 
experiment  ;  but  it  is  to  Professor  Stokes  that  we  are 
indebted  for  a  complete  series  of  researches  on  this  sub- 
ject. It  was  he  who  first  made  those  invisible  rays 
visible,  as  we  have  done." 

In  the  same  way  the  Professor  proceeded  to  show  that 
the  heat  rays  were  invisible  by  passing  a  beam  of  sun- 
light through  a  solution  of  iodine  in  spirits  of  wine, 
which,  although  it  completely  stopped  all  light,  allowed 
the  heat  rays  to  pass  iminterruptedly.  By  collect- 
ing these  invisible  rays  into  a  focus  by  means  of  a  lens, 
Dr.  Tyndall  was  enabled  to  ignite  various  combustible 
bodies. 

Thus  we  see  the  reason  why  certain  rays  produce 
certain  effects  on  the  eye,  each  particular  degree  of  re- 
fraction causing  a  different  set  of  vibrations,  resulting 
in  a  different  sensation  for  every  part  of  the  spectrum, 
and  reproducing  the  effect  of  various  colours  on  the 
optic  nerve.  In  the  following  chapters  we  shall  con- 
clude our  account  of  the  different  colours  in  the  spec- 
trum and  of  the  laws  of  light. 


106 


THIS  WONDERS  OF  OPTICS. 


CHAPTER  V. 

THE  LAWS  OF  REFLECTION.  MIRRORS. 

When  a  ray  of  light  falls  obliquely  on  any  polished 
surface,  as  that  of  a  mirror,  a  piece  of  water,  a  plate 
of  burnished  metal,  or  any  other  reflecting  substance, 
the  ray,  like  an  elastic  ball,  is  immediately  projected  in 
a  contrary  direction  to  that  in  which  it  fell.  Moreover, 
the  direction  in  which  it  is  reflected  is  at  right  angles 
to  the  surface,  and  in  the  same  plane  as  that  of  the  ray 
in  the  first  instance.  This  experiment  may  be  tried 
very  easily,  and  will  show  the  reason  for  the  two  fol- 
lowing laws. 

1.  The  angle  of  incidence  is  equal  to  the  angle  of  re- 
flection, and  vice  versa. 

2.  Reflection  can  only  take  place  in  one  direction — 
in  that  of  the  incident  rays,  both  of  which  are  always 
in  a  plane  perpendicular  to  the  reflecting  surface. 

The  following  figure  will  assist  the  student  in  per- 
forming experiments  on  the  reflection  of  light  from  flat 
surfaces. 

The  ray  A  B  falling  obliquely  on  the  horizontal 
mirror,  is  reflected  upwards  at  the  same  angle  in  the 
direction  b.  c.  This  may  be  proved  geometrically  by 
placing  a  graduated  circle  in  a  vertical  position  in  the 
plane  A  B  c,  when  we  shall  find  that  the  angle  A  B  D 
formed  by  A  B  (the  incident  ray)  with  the  perpendicular 
D  B  is  equal  to  the  angle  formed  by  this  perpendicular 
line  and  the  reflecting  ray  B  c.    You  may  also  prove 


THE  LAWS  OF  REFLECTION. — MIRRORS.  107 

in  the  same  way  that  these  three  lines  are  all  -in  the 
same  vertical  plane. 


Fia.  13— Reflection  from  Plane  Surfaces. 


Let  us  now  examine  the  effects  of  light  reflected 
from  plane  surfaces.  We  must  first,  however,  notice  a 
certain  optical  illusion  to  which  we  are  continually 
falling  a  prey,  almost  without  our  knowledge.  We 
always  fancy  objects  to  be  in  reality  .in  the  place  wh(re 
we  see  them,  and,  in  spite  of  our  having  already  enume- 
rated a  large  number  of  these  deceptions,  we  must 
still  add  one  more  to  the  list.  In  reality  we  rarely  see 
objects  in  the  place  where  they  really  are ;  for  if  by 
the  effect  ot  reflection,  refraction,  or  any  other  cause, 
the  rays  of  light  are  made  to  deviate  from  their  course, 
we  no  longer  see  the  object  from  which  they  proceed  in 
its  real  position,  but  in  the  direction  taken  by  the  lu- 
minous pencil  at  the  moment  of  entering  the  eye. 

For  instance,  if  the  ray  A  B  is  bent  during  its  passage 
to  the  eye  at  b,  and  consequently  reaches  it  in  the 


108 


THE  WONDERS  OF  OPTICS. 


direction  B  c,  it  is  at  A1,  and  not  at  A,  that  we  shall  see 
the  object  from  which  it  proceeds.  Ev^ry  ray  of  light 
which  passes  out  of  a  medium  of  a  certain  density  into 


Fig.  14. — Refraction 

another  of  a  different  density  is  bent  from  its  primary 
course,  or,  in  scientific  language,  it  is  refracted.  The 


Fig.  15. — Experimental  Proof  of  Refraction: 


experiments  we  made  in  a  former  chapter  on  the 
properties  of  the  prism  are  founded  on  this  principle. 


THE  LAWS  OF  REFLECTION. — MIRRORS.  109 


The  law  may  be  easily  illustrated  by  allowing  a  ray  of 
light  to  fall  upon  the  surface  of  a.  vessel  of  water,  as 
shown  in  the  preceding  figure. 

The  light  of  the  stars  and  planets  undergoes  a  similar 
deviation  when  passing  in  its  course  through  the  earth's 
atmosphere ;  and  at  the  moment  we  see  the  rising  of 
the  sun,  the  moon,  or  a  star,  they  are  in  reality  still 


Fig.  16.— The  Effects  of  Plane  Mirrors. 

below  the  horizon.  Our  eyes  consequently  are  still 
deceiving  us,  no  matter  what  part  of  the  domain  of  op- 
tics we  may  enter. 


THE  WONDERS  OF  OPTICS. 


There  are  two  kinds  of  mirrors — plane  and  curved. 
We  will  first  examine  the  properties  of  the  former  sort, 
being  those  which  are  ordinarily  applied  to  the  usages 
of  every-day  life. 

In  the  figure  in  the  preceding  page  we  have  a  young 
lady  looking  at  her  reflection  in  a  tall  cheval  glass. 
Every  point  upon  the  surface  of  her  clothes  and  face  is 
reflected  back  to  her  eye  from  the  surface  of  the  tin 
amalgam  which  has  been  applied  to  the  back  of  the  mir- 
ror by  the  looking-glass  maker,  for  the  purpose  of  ren- 
dering the  image  of  the  object  more  brilliant  than  if  the 
glass  alone  were  used.  The  rays  which  proceed  from 
every  one  of  these  points  strike  upon  the  surface  of  this 
metallic  layer,  are  stopped  by  its  opacity,  and  are  re- 


FiG.  17.— Reflection  from  the  Surface  of  Water. 

fleeted  back  to  the  eye  at  an  angle  equal  to  that  at  which 
they  strike  the  surface.  The  image  seen  by  the  eye  is 
formed,  consequently,  by  the  reflection  of  everv  o4  of 


THE  LAWS  OF  REFLECTION. — MIRRORS.  Ill 


these  rays ;  and  as  we  always  see  objects  in  the  direc- 
tion taken  by  the  luminous  ray  at  the  moment  it  enters 
the  eye,  we  fancy  we  see  objects  before  us  that  are 
really  behind,  or  on  each  side  of  us.  For  instance,  the 
ray  starting  from  the  left  foot  of  the  young  lady  in  the 
figure  is  reflected  from  the  point  indicated  on  the  sur- 
face of  the  glass,  but  the  eye  does  not  stop  here,  but 
sees  the  foot  at  an  equal  distance  beyond  the  mirror. 

The  same  thing  takes  place,  not  only  with  glass,  but 
with  all  substances  having  polished  surfaces.  Still  water, 
which  to  all  intents  and  purposes  has  a  polished  sur- 
face, reflects  the  objects  within  its  range  as  perfectly  as 
a  mirror. 

The  preceding  observations  apply  to  all  plane  re- 
flecting surfaces ;  but  there  are  other  sorts  of  mirrors, 
whose  effects  are  of  a  more  interesting  nature,  and 
which  we  must  hasten  to  describe — we  allude  to  those 
whose  surfaces  are  either  convex  or  concave. 

Curved  mirrors  are  made  of  a  great  variety  of  shapes, 
but  for  the  present  we  shall  only  describe  those  which 
are  spherical.  Spherical  mirrors  may  of  course  be 
either  concave  or  convex. 


Fig.  18.— Concave  Mirror. 

Suppose  the  arc  M  N  (fig.  18)  to  be  movable  round 
the  point  o,  this  revolution  will  describe  the  surface  of 
the  mirror.    The  central  point  c  of  the  hollow  sphere 


112 


THE  WONDERS  OF  OPTICS. 


of  which  the  mirror  forms  part,  is  called  the  centre  of 
curvature,  the  line  0  L  the  principal  axis.  By  remem- 
bering these  very  simple  definitions,  we  shall  be  able 
to  understand  the  action  of  these  mirrors  without  the 
slightest  difficulty. 

To  understand  how  the  rays  of  light  are  reflected 
from  the  surface  of  the  mirror  n  m  at  the  point  F,  which 
is  called  the  focus,  we  have  only  to  consider  the  mirror 
as  consisting  of  an  infinite  number  of  facets,  all  inclined 
towards  that  particular  point,  and  forming  by  reason  of 
their  immense  numbers  a  regular  spherical  surface.  In 
considering  the  mirror  from  this  point  of  view,  we  can 
immediately  see  that,  on  account  of  the  inclination  of 
the  supposed  facets,  the  rays  that  they  receive  are  all 
reflected  back  again  at  the  same  point ;  and  it  may  be 
proved  geometrically,  that  when  the  incident  rays  are 
parallel  the  focus  will  be  situated  somewhere  on  the  line 
0  c,  its  position  depending  on  the  curvature  of  the 
mirror. 

If,  therefore,  we  receive  on  a  spherical  mirror  a  pencil 
of  sunlight,  the  rays  which  compose  it  may  be  regarded 
as  parallel,  the  sun  being  at  so  great  a  distance  from  the 
earth ;  it  follows  that  these  rays  will  all  be  reflected 
together  in  a  particular  point,  viz.,  at  F,  and  if  any 
object  be  placed  there  it  will  be  illuminated  with  great 
brilliancy.  The  laws  governing  the  reflection  of  heat 
being  nearly  similar  to  those  regulating  the  action  of 
light,  the  rays  reflected  from  a  burning  body  will  ignite 
any  inflammable  substance  placed  at  the  point  F.  The 
focus  for  parallel  rays  is  called  the  principal  focus  of  a 
mirror.  Having  described  the  effects  of  parallel  rays, 
let  us  now  see  what  happens  when  the  source  of  light  is 
close  to  the  mirror.  If  it  is  placed  at  a  very  small 
distance,  the  luminous  rays  are  divergent  instead  of 
parallel,  and  their  meeting  point  becomes  changed  in 
accordance  with  the  laws  laid  down  at  the  beginning  of 


THE  LAWS  OF  REFLECTION. — MIRRORS.  113 


this  chapter.  That  is  to  say,  the  focus  will  approach 
more  or  less  to  the  centre  of  curvature  c,  according  as 
the  source  of  light  is  placed  nearer  to  or  further  from  the 
mirror;  consequently,  in  the  case  of  the  candle  in  fig. 
19,  instead  of  uniting  at  F,  the  rays  will  meet  at  /,  a 
point  situated  somewhat  nearer  the  mirror  than  the 
principal  focus.  If,  instead  of  placing  the  light  at  A,  we 
place  it  at/,  we  shall  find  the  rays  will  be  concentrated 
at  the  point  A.  Thus  the  foci  are  consequently  related 
to  each  other,  and  are  hence  called  conjugate  foci.  It 
will  be  readily  seen  that  a  spherical  mirror  may  have  an 
infinite  number  of  conjugate  foci,  according  to  the  dis- 
tance of  the  source  of  light.  It  is  also  clear,  that  if  we 
cause  the  light  to  approach  the  mirror,  the  focus  will 
also  approach  it. 


Fig.  19.— Conjugate  Foci. 

Continuing  our  experiment,  we  shall  find  that  when 
the  candle  passes  the  principal  focus  so 'as  to  be  between 
it  and  the  mirror,  the  reflected  rays  first  become  parallel 
and  then  divergent,  and  cannot  consequently  produce 
any  focus  beyond  the  mirror,  but  are  reflected  in  the 
way  shown  in  fig.  20. 

In  experimenting  on  the  plane  mirror,  we  imagined 
we  saw  the  object  at  a  certain  distance  behind  it ;  the 
same  thing  happens  when  we  see  ourselves  reflected  in 


114  THE  WONDERS  OF  OPTICS. 

a  concave  mirror,  and  the  particular  point  at  which  we 
suppose  we  see  our  reflection  is  called  the  virtual  focus. 


Fig.  20.— Virtual  Focua. 

If  instead  of  a  candle  we  place  our  head  before  a 
concave  mirror,  we  shall  see  ourselves  magnified  as  in 
fig.  21. 


Fig.  21. — Concave  Mirror, 


We  shall  easily  see  how  this  happens  by  tracing  the 
paths  of  the  rays  in  fig.  22. 


THE  LAWS  OF  REFLECTION. — MIRRORS.  115 


Fig.  23.— The  Reversal  of  real  Images. 


116 


THE  WONDERS  OF  OPTICS. 


eye  in  such  a  way  as  to  appear  to  proceed  from  a  point 
beyond  the  mirror,  A.  In  the  same  manner  the  rays  re- 
flected from  the  chin  appear  to  take  their  origin  from 
the  point  B.  If,  on  the  other  hand,  we  place  ourselves 
at  a  distance  from  the  principal  focus,  we  shall  produce 
a  reversed  and  diminished  image  of  our  face.  This 
image  is  not  illusory,  like  the  preceding  ones,  but  is 
real,  and  may  be  received  upon  a  screen,  as  shown  in 
fig.  23. 

We  may  easily  follow  the  path  of  the  rays  as  shown 
in  the  figure,  and  we  shall  see  that  the  rays  forming 
the  images  of  the  church-tower  and  the  terrace  below, 
cross  at  a  certain  point. 

Convex  mirrors  produce  precisely  opposite  effects,  and 
give  a  diminished  image  instead  of  a  magnified  one,  as 
may  be  perceived  on  examining  fig.  24„ 


Fia.  24.— Diminishing  power  of  Convex  Mirrors. 


METALLIC  BURNING  MIRRORS. 


117 


*    CHAPTER  VI. 

METALLIC  BURNING  MIRRORS. 

The  classical  student  will  remember  that  Archimedes 
burned  the  fleet  of  Marcellus,  by  means  of  burning- 
glasses,  from  the  heights  of  the  fortifications  of  his 
native  city  of  Syracuse.  Unfortunately,  any  account  of 
the  system  of  catoptrics,  or  the  science  of  reflections, 
employed  by  the  ancient  Syracusan  in  their  con- 
struction is  lost  to  us,  and  many  modern  writers  have 
gone  so  far  as  to  doubt  the  fact  altogether.  The 
knowledge  of  the  properties,  however,  of  concave 
mirrors  which  we  have  just  been  acquiring,  will  enable 
us  to  form  a  pretty  good  guess  as  to  the  means  adopted 
by  Archimedes  for  the  destruction  of  the  enemy's  fleet. 
The  ancients,  not  having  the  means  of  either  casting  or 
grinding  such  enormous  mirrors,  must  have  constructed 
them  of  a  large  number  of  small  ones,  so  arranged 
that  the  images  of  the  sun  reflected  by  them  would  all 
fall  in  the  same  place,  or  nearly  so.  In  this  case,  the 
larger  the  number  of  mirrors,  the  greater  would  be  the 
burning  effect.  In  order  to  explain  the  reflection  of 
rays  incident  upon  the  surface  of  concave  mirrors,  we 
supposed  them  to  consist  of  an  immense  number  of 
plane  mirrors  placed  in  a  curve,  so  that  the  reflected 
rays  might  all  meet  in  one  point ;  but  on  examining 
into  the  history  of  burning  mirrors,  we  find  that  the 
plan  has  been  adopted  in  reality  in  a  great  number  of 


118 


THE  WONDERS  OF  OPTICS. 


instances.  We  have  also  said,  that  the  reflection  of  the 
heating  rays  was  governed  by  similar  ]aws  to  those 
influencing  the  rays'  of  light ;  consequently,  by  directing 
a  pencil  of  sunlight  upon  the  surface  of  a  concave 
mirror,  we  obtain  the  maximum  of  light  and  heat  at 
the  focal  point. 

Many  modern  writers  give  the  ancients  too  little 
credit  for  their  knowledge  of  optical  principles,  and 
late  investigations  seem  to  prove  that  the  old  school  of 
philosophers  were  much  more  learned  in  these  matters 
than  has  been  generally  supposed.  The  discovery  of  a 
rock  crystal  double  convex  lens  in  an  Egyptian  tomb  of 
great  antiquity  is  an  instance  of  this.  Descartes  wrote 
a  little  treatise  to  prove  that  the  stories  related  of  the 
burning  mirrors  of  Archimedes  were  pure  fabrications, 
although  many  Latin  authors  have  described  them  both  as 
being  used  by  that  philosopher  and  in  more  modern 
times  ;  Dion,  for  instance,  who  lived  in  the  early  part 
of  the  sixth  century,  states  that  at  the  siege  of  Con- 
stantinople, Proclus  burnt  the  fleet  of  Vitalian  with 
mirrors  of  brass ;  but  the  opinion  of  Descartes  seemed 
to  outweigh  all  other  testimony.  BufFon,  who  wished 
to  sift  the  matter  thoroughly,  constructed  for  himself, 
after  many  previous  experiments  on  the  laws  of  reflec- 
tion, a  series  of  mirrors  that  closely  imitated  those 
ascribed  to  Archimedes.  His  first  memoir,  "  On  the 
Invention  of  Mirrors  capable  of  burning  at  a  great  Dis- 
tance/' was  published  in  the  Transactions  of  the  French 
Academy  of  Sciences  for  1747.  A  few  years  later  he 
combated  both  theoretically  and  practically  the  opinion 
of  Descartes,  in  ;  memoir  containing  an  account  of  an 
immense  number  of  experiments.  Before  speaking  of 
the  extraordinary  effects  of  burning  mirrors,  it  will  be 
as  well  to  do  justice  to  the  predecessors  of  the  learned 
naturalist  we  have  just  mentioned,  by  quoting  a  passage 
from  the  works  of  Father  Kircher,  who,  128  years  pre- 


METALLIC  BURNING  MIRRORS. 


119 


viously,  experimented  in  this  direction  with  great  pa- 
tience and  perseverance,  and  tried  to  prove  that  the 
stories  related  of  Archimedes  were  true.  "  The  larger 
the  surface  of  a  mirror,"  says  this  philosopher  (who,  like 
Huyghens,  was  a  practised  astronomer),  u  the  more  light 
it  reflects  from  the  objects  opposite  to  it.  If  it  is  only 
a  foot  square,  it  will  throw  a  square  foot  of  light  upon 
any  wall  or  screen  placed  before  it.  Experiment  shows 
that  this  light  is  composed  of  an  infinite  number  of  rays 
reflected  from  different  points  on  the  surface  of  the  mir- 
ror. Direct  the  rays  from  a  second  mirror  upon  the 
same  place  as  those  from  the  first,  and  the  light  and 
heat  will  clearly  be  doubled.  They  will  become  trebled 
if  you  direct  the  rays  from  a  third  mirror  upon  the 
same  spot,  and  so  on  ad  infinitum.  In  order  to  prove 
that  the  intensity  of  the  light  and  heat  is  in  direct  pro- 
portion to  the  number  of  reflecting  surfaces  employed, 
I  took  five  mirrors,  and  found  that  on  exposing  them  to 
the  sun  I  obtained  with  only  one,  less  heat  and  light 
than  if  I  used  direct  sunlight.  With  two  the  light  and 
heat  increased  considerably  ;  three  gave  as  much  heat 
as  an  ordinary  fire,  and  four  gave  me  a  still  greater 
effect.  I  therefore  concluded  that  by  multiplying  these 
plane  mirrors,  I  not  only  obtained  greater  effects  than 
those  got  by  using  parabolic,  hyperbolic  or  elliptical  mir- 
rors, but  that  I  could  use  them  upon  objects  at  a  much 
greater  distance.  With  five  mirrors  I  could  obtain 
these  effects  at  a  distance  of  100  feet,  but  what  terrible 
phenomena  would  have  taken  place  had  I  used  one 
thousand  instead  of  five?"  He  ends  by  begging  math- 
ematicians to  experiment  in  this  direction  with  greater 
care  than  they  had  hitherto  done. 

After  Kircher  we  may  cite  as  an  experimentalist  with 
these  terrible  instruments  the  French  philosopher  Vil- 
lette,  who  constructed  several  mirrors,  in  direct  imita- 
tion of  those  of  Archimedes,  for  Louis  XIV.  and  other 


120 


THE  WONDERS  OF  OPTICS. 


sovereigns.  The  Journal  des  Savants  for  1679  gives 
an  account  of  his  principal  metallic  burning  mirror  in 
the  most  eulogistic  terms,  adding  an  instance  of  igno- 
rance which  is  singularly  quaint  and  curious.  It  is  of 
the  fourth  and  most  perfect  of  Villette's  mirrors  that 
the  Journal  des  Savants  speaks,  the  first  having  been 
bought  by  Tavernier,  and  presented  to  the  Shah  of 
Persia,  who  considered  it  as  one  of  the  rarest  and  most 
precious  curiosities  that  he  possessed :  the  second  was 
sold  to  the  King  of  Denmark,  and  the  third  was  given 
by  M.  Villette  to  Louis  XIV.,  from  whom  he  received 
the  praises  and  rewards  that  were  due  to  his  talent  and 
perseverance.  "  It  was  thirty-four  inches  in  diameter, 
and  vitrified  flints  and  bricks  almost  instantaneously, 
no  matter  how  large  they  were.  It  consumed  the  green- 
est wood,  burning  it  to  ashes  in  an  instant,  and  fused 
the  most  refractory  metals  with  equal  ease  and  quick- 
ness. Steel,  no  matter  how  hard,  resisted  its  power  no 
more  than  other  metals,  and  melted  so  quickly  that  one 
part  burnt  away  in  inconceivably  brilliant  sparks,  some 
of  them  forming  stars  as  large  as  a  franc  piece,  leaving 
a  flowing  mass  of  metal  behind.  The  last  made  by 
Villette  was  still  more  powerful,  being  larger  and  more 
carefully  made.  It  was  forty-four  inches  in  diameter, 
and  three  inches  and  a  line  deep.  Its  burning  point,  or 
focus,  was  situated  at  a  distance  of  three  feet  seven 
inches  from  the  surface,  and  was  apparently  as  large  as 
a  five-sou  piece ;  and  it  was  at  this  spot,  where  the  rays 
of  light  and  heat  were  concentrated  into  so  small  a 
space,  that  the  wonderful  effects  of  its  violent  power 
became  manifest,  the  spot  of  light  being  of  such  bril- 
liancy that  the  eyes  could  no  more  withstand  its  bright- 
ness than  that  of  the  sun.  Besides  the  property  of 
burning  which  it  possessed  in  so  wonderful  a  degree,  it 
was  capable  of  exhibiting  other  effects  just  as  curious 
as  those  already  related.    It  had  the  power  of  sending 


METALLIC  BURNING  MIRRORS 


121 


the  images  of  objects  to  a  distance  of  fifteen  feet  or 
more,  so  that  a  man  looking  at  himself  in  this  mirror 
with  a  sti^k  or  sword  in  his  hand,  saw  the  image  of  them 
suspended  in  the  air,  apparently  ready  to  strike  the  ob- 
server. On  seeing  such  an  effect  for  the  first  time,  the 
observer  could  hardly  fail  to  experience  the  greatest 
surprise,  and  even  fear;  and  it  is  stated  that  the  king 
having  placed  himself,  sword  in  hand,  before  one  of 
these  mirrors,  in  order  to  observe  the  effect,  was  sur- 
prised to  find  himself  face  to  face  with  an  armed  hand 
apparently  directed  against  him.  When  he  advanced, 
the  hand  seemed  to  spring  forward  to  meet  him.  The 
king  could  not  conceal  his  sin  prise  and  fright,  and  after- 
wards felt  so  ashamed  at  being  terrified  with  a  mere 
shadow  that  he  ordered  the  mirror  to  be  taken  away, 
and  could  never  be  prevailed  upon  to  look  into  it  again." 
The  Journal  des  Savants  then  goes  on  quaintly  to  re- 
mark on  the  various  startling  effects  produced  by  these 
mirrors,  winding  up  by  stating  that  its  powers  of  reflec- 
tion were  so  great,  that  at  night  the  light  of  a  torch  or 
flambeau  was  reflected  so  perfectly  that  an  observer 
placed  at  four  hundred  feet  distant  could  read  the  small- 
est print. 

It  also  mentions  a  curious  piece  of  superstition  on 
the  authority  of  a  scientific  writer  of  the  name  of  Rob- 
ertson, who  states  that  it  happened  at  Liege.  In  read- 
ing the  accounts  of  these  experiments  we  can  see  how 
easily  the  minds  of  individuals  were  affected  in  those 
days  by  the  wonderful.  It  happened  while  one  of  Vil- 
lette's  mirrors  was  at  Li6ge,  that  the  latter  end  of  the 
summer  was  somewhat  rainy,  and  great  fears  were  en- 
tertained that  a  bad  harvest  and  dear  bread  would  be 
the  result.  Certain  evil-minded  people,  who  had  taken 
a  fancy  to  the  mirror  and  wished  to  possess  it  by  unfair 
means,  spread  the  report  that  the  continual  rain  was 
entirely  caused  by  its  action  on  the  clouds  and  sun,  and 


122 


THE  WONDERS  OF  OPTICS. 


that  the  coming  famine  must  be  laid  upon  the  shoulders 
of  its  owner  and  inventor.  This  absurd  idea  took  such 
forcible  possession  of  the  minds  of  the  populace  of 
Liege,  that  great  mobs  collected  together,  uttering  all 
kinds  of  maledictions  against  the  mirror  and  its  inven- 
tor, and  at  last  became  so  violent  that  they  attacked 
Villette's  house  with  the  intention  of  smashing  his  great 
wcrk,  and  administering  to  the  unfortunate  philosopher 
the  shastisetnent  they  supposed  he  deserved.  Happily, 
however,  for  M.  Villette  and  his  mirror,  Liege  was 
governed  in  those  days  by  the  Prince  Bishop  of  Co- 
logne, who  was  a  man  of  great  enlightenment.  He 
put  the  crowds  round  M.  Villette's  to  flight  by  armed 
force,  but  he  found  that  the  conviction  that  all  the  coming 
mischief  would  result  from  the  unlucky  mirror  was 
so  strong,  that  he  was  obliged  to  issue  a  pastoral  pe- 
remptorily declaring  that  the  idea  had  originated  with  a 
number  of  malicious  people,  who  spared  no  pains  to 
propagate  it  for  their  own  bad  purposes,  and  that  it 
w  m  a  mischievous  and  dangerous  error  to  ascribe  to 
a  mirror  a  power  which  only  belonged  to  the  Almighty. 

In  1747,  Buffon  performed  many  extraordinary  ex- 
periments with  burning  mirrors,  which  were  more  sur- 
prising than  any  that  had  hitherto  been  described. 
They  were  mostly  performed  at  the  Jar  din  des  Planter, 
at  Paris,  of  which  institution  Buffon  was  director ;  and 
many  of  them  are  worth  describing. 

On  the  3rd  of  April,  at  about  two  o'clock  in  the  af- 
ternoon, the  great  mirror  was  mounted  on  its  stand,  and 
was  found  to  be  capable  of  setting  a  plank  of  wood  on 
fire  at  a  distance  of  138  feet,  when  128  glasses  were 
used,  although  the  light  was  weak  at  the  time,  and  the 
sun  was  covered  with  mist.  In  pursuing  these  experi- 
ments great  care  had  to  be  taken  to  prevent  the  by- 
standers placing  themselves  within  range  of  its  terri- 
ble power,  for  several  were  nearly  blinded  by  looking 


METALLIC  BURNING  MIRRORS. 


125 


at  the  brilliant  focal  point  of  the  instrument.  The 
next  day,  at  eleven  in  the  forenoon,  although  the  sun 
was  still  covered  with  mist  and  fleecy  clouds  they  were 
able  to  produce  such  a  heat  at  150  feet  distant,  with 
154  glasses,  that  a  pitched  plank  began  to  smoulder 
and  would  have  burnt  into  flame  had  not  the  sun  disap- 
peared at  that  particular  moment.  On  the  fifth  of 
April,  at  three  in  the  afternoon,  with  the  light  much  in 
the  same  weak  condition  as  it  was  on  the  other  days, 
they  succeeded  in  igniting  at  150  feet  distant,  a  heap  of 
shavings  of  deal  mixed  with  charcoal  and  sulphur, 
in  less  than  a  minute  and  a  half,  with  154  glasses. 
When,  however,  the  sun  shone  with  its  natural  power, 
a  few  seconds  were  sufficient  to  effect  these  results. 

On  the  10th,  when  the  sun  was  shining  pretty  power- 
fully, a  pitched  pine  plank  was  easily  fired  with  128 
glasses,  at  150  feet  distant.  In  this  case  the  ignition 
was  very  sudden,  and  extended  over  the  whole  of  the 
radiant  spot  forming  the  focus,  which  at  the  distance 
named  measured  16  inches  in  diameter.  The  same  day 
at  half-past  two,  a  pitched  elm  plank  covered  in  some 
places  with  chopped  wood,  was  set  fire  to  with  extreme 
rapidity,  and  burnt  with  such  violence  that  it  had  to 
be  dipped  in  water  before  it  could  be  put  out.  In 
this  experiment  148  glasses  were  used,  at  a  distance  of 
150  feet. 

On  the  11th  of  April,  the  burning  point  was  fixed  at 

20  feet  distant  from  the  mirror,  and  combustible  sub- 
stances were  easily  burnt  with  only  12  glasses.  With 

21  glasses  a  half-burnt  elm  plank  was  set  fire  to,  and 
with  45  a  piece  of  tin  weighing  six  pounds  was  almost 
immediately  melted.  Silver  sheet  was  fused,  and  an 
iron  plate  was  made  red-hot  with  117  glasses.  In 
giving  an  account  of  these  interesting  experiments, 
Buffon  expresses  his  conviction  that  at  50  feet  it  would 
have  been  easy  to  have  melted  metals  if  all  the  glasses 


126 


THE  WONDEKS  OF  OPTICS. 


of  the  mirrors  had  been  used.  When  used  at  that  dis- 
tance, the  burning  spot  was  six  to  seven  inches  in 
diameter.  He  also  noticed  that  when  metals  were 
melted,  part  of  them  were  dissipated  in  brilliant  vapour, 
which  was  so  thick  as  to  cast  a  shadow  on  the  ground, 
although  it  seemed  to  be  as  bright  as  the  sun  itself. 
When  the  sun  was  at  its  full  strength,  and  all  the 
glasses  were  brought  into  requisition,  wood  was  set  on 
fire  at  a  distance  of  over  200  feet,  and  metals  and  mine- 
rals were  fused  at  40  and  50  feet.  Hence  the  possi- 
bility of  making  and  using  these  mirrors  as  Archimedes 
was  said  to  have  done,  was  proved  practically  by  the 
great  naturalist.  Fig.  25  represents  a  burning  mirror 
in  action. 

Robertson,  an  English  philosopher,  residing  in  France 
during  the  days  of  the  first  Republic,  reconstructed 
the  mirrors  described  by  historians  as  being  used  by 
Archimedes,  and  the  results  he  obtained  were  thought 
sufficiently  important  by  the  Council  of  the  Department 
of  Ourthe  to  merit  an  attentive  examination  by  two 
members  of  their  body,  who  reported  in  favour  of  their 
being  used  as  instruments  of  war. 

It  would  be  possible  to  pursue  this  subject  still  fur- 
ther, and  give  an  account  of  numerous  experiments 
made  on  burning  mirrors  by  various  philosophers,  but 
we  must  not  forget  that  it  is  light  and  heat  that  we 
have  more  especially  to  deal  with  in  the  present  work. 
Already  we  have  possibly  strayed  from  our  path  a  little 
too  far,  but  the  two  influences  are  so  closely  connected 
with  each  other  that  it  is  almost  impossible  to  speak  of 
them  separately  when  reflection  is  in  question. 


LENSES. 


127 


CHAPTER  VII. 

LENSES. 

The  word  lens  is  derived  from  the  Latin  name  of  the 
seed  of  the  Ervum  lens,  or  ordinary  lentil.  When  eat- 
ing this  wholesome  vegetable,  almost  every  one  has  no- 
ticed that  its  shape  is  exactly  that  of  a  double  convex 
lens,  as  represented  in  the  following  figure : — 


Fig.  26. — Double  Convex  Lens. 

Perhaps  it  would  be  more  correct  it  we  were  to  say  that 
a  double  convex  lens  is  like  a  lentil,  rather  than  turn 
the  comparison  the  other  way,  seeing  that  this  little 
seed  has  given  its  name  not  only  to  the  particular- 
shaped  glass  depicted  above,  but  also  to  some  five  others 
more  or  less  analogous  to  it. 

In  fig.  27  we  have  the  different  forms  of  lenses 


128  THE  WONDERS  OF  OPTICS. 

shown  in  section.  The  first  is  the  double  convex  lens, 
the  second  the  plano-convex,  the  third  and  sixth  the 
concavo-convex,  the  fourth  the  double  concave,  and  the 


14  9  L  6  * 

Fia.  27.— Forms  of  Lenses. 

fifth  the  plano-concave.  A  crossed  lens  is  a  double 
convex  lens  whose  one  side  is  more  convex  than  the 
other.    The  third  lens  is  also  called  meniscus. 

The  properties  of  the  first,  second,  and  third  are 
similar;  that  is  to  say,  they  cause  parallel  rays  of  light 
passing  through  them  to  converge  at  a  certain  point, 
called  their  focus  ;  while  the  three  others  have  a  diver- 
gent action  on  rays  passing  through  them.  By  exa- 
mining the  path  of  the  rays  through  these  lenses,  we 
shall  find  that  the  first  three  magnify  objects  seen 
through  them,  while  the  latter  have  the  contrary  effect. 

As  in  the  case  of  the  curved  mirrors,  the  rays  falling 
on  the  surface  of  a  convex  lens  may  be  either  parallel, 
divergent,  or  convergent.  In  the  case  of  parallel  rays, 
as  depicted  in  the  following  figure,  they  are  represented 
as  meeting  at  a  point  beyond  the  lens,  which  is  called 
the  sidereal  focus,  or  the  focus  for  parallel  rays.  It  is 
generally  found  by  causing  the  image  of  the  sun  or  of 
some  distant  object  to  be  thrown  by  the  lens  upon  a 
screen,  or  by  knowing  the  curvature  of  the  faces,  and 
the  refractive  power  of  the  glass. 

Every  ray  on  striking  the  surface  of  the  lens  is  re- 
fracted inwards,  until  it  meets  with  its  companions  at 


LENSES.  129 

the  focus  F,  in  accordance  with  the  law  of  refraction, 
by  which  a  ray  of  light  passing  from  one  transparent 
medium,  such  as  air,  to  another  which  in  this  instance 
is  glass,  becomes  refracted  or  bent  in  proportion  to  the 


Fig.  28. — Path  of  a  Ray  through  a  Convex  Lens. 

relative  density  of  the  two  mediae.  The  nearer  the  ray 
passes  to  the  edge  of  the  lens,  the  more  it  is  refracted, 
the  angle  of  incidence  being  greater ;  the  ray  through 
the  exact  centre  being  uninfluenced  by  the  form  of  the 
glass.  Hence  they  all  meet  in  a  single  point.  Figs. 
29  and  30  show  the  path  of  the  rays  when  they  are 
divergent  and  convergent. 


Fig.  29.— Path  of  divergent  Rays  through  a  Convex  Lens. 

If  the  rays  of  light  are  not  parallel,  as  in  the  case 
of  the  source  of  light  being  near  the  lens,  they  do  not 
converge  so  rapidly  as  when  they  proceed  from  a  dis- 
tant object,  consequently  the  focus  for  near  objects  is 
longer  in  proportion  to  their  distance.    In  fig.  29  for 


130 


THE  WONDERS  OF  OPTICS. 


instance,  if  a  candle  be  placed  as  shown,  and  a  screen 
on  the  other  side  of  the  lens,  a  point  will  be  found 
where  the  image  of  the  candle  is  seen  upon  it  in  a  re- 
versed position.  The  distance  between  these  two 
points  is  always  relative,  and  they  are  called  conjugate 
foci.  Thus,  the  candle  may  change  places  with  the 
screen  with  a  similar  effect,  as  long  as  the  exact  position 
of  the  two  points  is  preserved.    If  the  candle  is  placed 


Fig.  30.— Conjugate  Foci. 

farther  off,  we  mast  diminish  the  distance  between  the 
screen  and  the  lens,  and  vice  versa.  In  fact,  the  nearer 
the  object,  the  longer  the  focus  ;  the  farther  it  is  off, 
the  shorter  the  focus.  Half  an  hour's  experiment 
with  a  double  convex  lens,  a  piece  of  white  card-board, 
and  a  small  candle,  will  teach  the  student  more  about 
the  proparties  of  convex  lenses  than  a  chapter  of  ex- 
planation. A  conmon  magnifying  glass,  or  even  an 
old  spectacle  lens,  will  serve  the  purpose  of  more  ex- 
pensive instruments. 

We  now  procaai  to  speak  of  the  images  formed 
by  lenses.  In  fig.  31  we  have  a  flower  placed  on  one 
side  of  a  lens.  As  it  is  not  at  an  infinite  distance,  the 
rays  sent  out  by  its  various  parts  are  convergent,  and 
not  parallel,  consequently  they  do  not  meet  at  the 
sidereal  focus,  but  at  a  point  beyond  it,  according  to  the 
rule  already  laid  down.  The  rays  proceeding  from  the 
exact  centre  of  the  flower  striking  the  lens  exactly  in 


LENSES. 


131 


the  middle  at  right  angles,  suffer  no  change,  the  others 
being  refracted  in  proportion  to  their  angles  of  inci- 
dence. 


F'G.  31. — Images  formed  by  Convex  Lenses. 


The  rays  proceeding  from  the  flower  cross  each  other 
ut  a  certain  point:  hence  the  image  on  the  screen  is 
reversed.  The  dimensions  of  the  image  will  depend  on 
the  distance  of  the  object  from  the  lens.  This  is  a  fact 
we  meet  with  every  day,  when  using  an  opera-glass  or 
a  telescope.  Images  formed  by  convex  lenses  upon  a 
screen  are  called  by  opticians  real  images,  in  contradis- 
tinction to  those  which  are  the  result  of  mere  reflection, 
as  in  the  case  of  plane  mirrors.  These  latter  are  known 
as  virtual  images  and  are  produced  by  convex  lenses 
as  well  as  by  plain  reflecting  surface  s.  In  fig.  32,  for 
instance,  the  unreversed  image  of  the  insect  seen  by  the 
eye  is  not  a  real  image,  but  a  virtual  one, — a  fact  that 
might  be  easily  proved  by  placing  a  screen  in  the  posi- 
tion of  the  eye,  when  it  would  be  found  that  no  image 
would  be  formed. 

When  using  an  ordinary  magnifying-glass  we  see  the 
virtual  image  of  the  object  we  are  looking  at,  but  in  the 
case  of  a  telescope  or  opera-glass  we  see  the  real  image 
of  the  object,  formed  by  the  large  lens  in  front,  and 


132  THE  WONDERS  OF  OPTICS. 

reversed  again  by  the  arrangement  of  small  lenses  next 
to  the  eye. 


Fig.  32.— Magnifying  Property  of  Convex  Lenses 


Double  concave  lenses  produce  effects  which  are  just 
the  reverse  of  those  we  have  been  considering.  Instead 
of  increasing  in  thickness  from  the  edges. to  the  centre, 
they  follow  the  contrary  plan,  and  increase  from  the 
centre  to  the  edges.  Consequently,  instead  of  the  rays 
meeting  at  the  focus,  they  diverge  from  each  other,  and 
gradually  spread  out,  as  shown  in  fig.  33. 


A 


B 


•    Fig.  33.— Diminishing  Effect  of  Concave  Lenses 

The  above  figure  shows  the  path  of  the  rays  proceeding 
from  the  vase,  and  meeting  the  eye  at  such  an  angle 


134 


LENSES. 


135 


that  the  virtual  image  is  greatly  diminished.  Concave 
lenses,  as  the  student  has  no  doubt  already  guessed,  do 
not  give  real  images. 

The  effects  produced  by  the  action  of  concave  mirrors 
may  be  produced  with  just  as  much  facility  by  convex 
lenses.  If  a  body  is  placed  in  a  focus  of  a  lens  which 
receives  the  direct  rays  of  the  sun,  the  heat  as  well  as 
the  light  will  be  concentrated  at  one  point ;  and  if  the 
object  is  combustible,  it  will  take  fire  sooner  or  later, 
according  to  the  size  of  the  lens.  All  the  experiments 
mentioned  Ly  Bulfon  as  being  produced  by  a  concave 
mirror  are  equally  obtainable  with  a  concave  lens. 
When  of  sufficient  diameter,  the  most  refractory  metals, 
such  as  platinum  or  iridium,  may  be  melted  and  dissi- 
pated into  vapour.  Before  lucifer  matches  and  vesu- 
vians  were  as  common  as  they  are  now,  it  was  not  at 
all  unusual  to  find  smokers  carrying  a  small  burning- 
glass  and  a  piece  of  tinder,  for  the  purpose  .of  lighting 
their  pipes  or  cigars;  and  there  hardly  exists  a  boy  who 
has  not  lighted  a  bonfire  in  the  fields  or  playground  by 
means  of  an  old  spectacle  lens  or  telescope  glass. 

Amongst  other  applications  of  this  property  of  lenses 
may  be  mentioned  that  of  causing  guns  to  fire  at  a 
certain  time,  by  arranging  a  small  burning-glass  above 
the  touch-hole.  In  the  Gardens  of  the  Palais  Royal,  at 
Paris,  there  is  such  a  gun,  so  arranged  that  on  sunny 
days  it  fires  exactly  at  noon,  or,  in  other  words,  at  the 
moment  the  sun  comes  to  the  meridian.  Every  fine 
day  towards  twelve  o'clock,  crowds  of  Parisians  who 
have  nothing  to  do  may  be  seen  bending  their  steps 
towards  the  Palais  Royal  to  set  their  watches  by  the 
gun,  wThich  they  believe  to  be  superior  as  a  time-keeper 
to  the  finest  chronometer  in  the  world.  There  they 
stand,  most  of  them  old  fellows  with  a  scar  or  two  about 
their  faces,  showing  that  they  have  nobly  won  the  rest 
they  appear  to  enjoy  so  innocently  and  calmly  with 


V6H 


THE  WONDERS  OF  OPTICS. 


watch  in  hand,  leaning  against  the  railings,  and  waiting 
with  impatience  the  moment  when  true  solar  noon  is 
indicated  by  the  sharp  report  of  the  little  piece.  Their 
belief  in  the  correctness  of  solar  time  is  something 
astonishing;  and  if  a  bystander  were  to  insinuate,  rio 
matter  how  delicately,  that  solar  time  varied  slightly 
every  now  and  then,  he  would  either  receive  a  smile  of 
pitying  contempt,  or  else  he  would  be  called  out  upon 
the  spot.  Fig.  34  gives  a  pretty  view  of  the  celebrated 
cannon  of  the  Palais  Royal. 

We  now  come  to  another  application  of  the  refract- 
ing power  of  lenses,  in  the  way  of  concentrating  rays, 
which  is   infinitely  more  valuable  to  humanity  than 


Fig.  35.— FresnePs  Lighthouse  Apparatus. 

either  of  those  we  have  just  mentioned  ;  we  mean  the 
construction  of  enormous  refracting  apparatuses  for 
lighthouse  purposes.    The  first  lighthouse  of  which  we 


LENSES. 


137 


have  any  record  is  that  which  was  erected  on  the  island 
of  Pharos,  by  Ptolemy  Philadelphia,  in  the  year  470 
of  the  foundation  of  Rome.  This  was  merely  a  tower, 
upon  the  top  of  which  fires  were  kept  burning  at  night; 
but  as  the  world  progressed,  the  blazing  tar-barrel  or 
wood  fire  gave  place  to  the  carefully-constructed  lamp 
and  silvered  reflector  apparatus,  which  are  fast  disappear- 
ing in  their  turn  before  the  electric  or  Drummond  light 
and  the  refracting  apparatus  constructed  by  Fresnel, 
who  was  the  first  to  endeavour  to  abolish  the  old- 
fashioned  and  inefficient  metallic  mirror  from  the 
lanterns  of  lighthouses.  Fig.  35  shows  a  section  of 
Fresnel's  apparatus.  A  is  a  plano-convex  lens  of  about 
a  foot  in  diameter,  whose  focus  corresponds  with  those 
of  the  concentric  lenticular  rings  of  glass  which  sur- 
round it,  and  which  are  seen  more  plainly  in  fig.  36. 
These  rings,  which  are  ground  and  polished  with  the 
greatest  accuracy,  are  somewhat  in  the  shape  of  an 
ordinary  quoit,  and  are  equivalent  to  a  plano-convex 
lens  with  the  centre  portion  cut  out.  This  arrangement 
is  so  powerful  that  the  distance  at  which  a  light  pro- 
vided with  it  can  be  seen  is  only  limited  by  bad  weather, 
the  state  of  the  atmosphere  and  the  distance  of  the 
horizon.  It  is  common  for  such  lights  to  be  seen  at  a 
distance  of  between  fifty  and  sixty  miles.  The  ap- 
paratus is  mostly  arranged  in  the  form  of  an  octagon, 
and  is  generally  provided  with  additional  reflecting 
mirrors  at  those  parts  above  the  light  which  are  out  of 
the  range  of  the  lenses.  The  light  shining  fully  in  eight 
directions  at  one  time,  can  scarcely  be  missed  by  any 
hhip  within  range;  but  in  order  to  guard  against  any 
possibility  of  accident,  the  optical  apparatus  is  often 
made  to  revolve  by  clockwork,  so  that  every  point  of 
the  ocean  is  illuminated  in  turn.  By  using  coloured 
glasses,  or  by  causing  the  light  to  disappear  at  distinct 
intervals,  different  lighthouses  may  be  identified  by 


138 


THE  WONDERS  OF  OPTICS. 


ships  that  are  out  of  their  reckoning.  Fig.  36  repre- 
sents the  interior  of  the  lantern  of  a  first  class  light- 
house, showing  the  arrangement  of  the  lenticular  rings 
round  the  central  lens.  If  ever  the  student  should 
pass  through  Havre,  he  should  not  miss  the  oppor- 
tunity of  seeing  this  noble  apparatus,  which  is  one  of 
the  finest  ever  manufactured* 


FiG.  36.— Lantern  of  a  First-class  Lighthouse. 


140 


OPTICAL  INSTRUMENTS. 


141 


CHAPTER  VIII. 

OPTICAL  INSTRUMENTS. — THE  SIMPLE  AND  COMPOUND 
MICROSCOPE.  THE  SOLAR  AND  PHOTO-ELECTRIC  MI- 
CROSCOPE. 

The  lenses  and  mirrors  whose  properties  we  have 
been  considering  in  the  previous  chapters,  have  been 
combined  in  different  ways  for  the  purpose  of  exami- 
ning objects  too  small  or  too  distant  to  be  perceived  by 
the  human  eye.  To  instruments  used  for  the  former 
purpose  the  name  of  microscope  has  been  given,  from 
two  Greek  words  signifying  small  and  to  see.  In  like 
manner  the  name  of  telescope  is  also  derived  from  two 
Greek  words,  meaning  distant  and  to  see.  Besides 
these  two  classes  of  optical  instruments,  others  have 
been  devised  to  facilitate  the  depicting  of  natural 
objects,  either  by  means  0f  the  pencil  or  of  photo- 
graphy, or  to  amuse  the  eye  by  optical  illusions.  Thus 
we  have  the  camera  obscura,  the  camera  lucida,  the 
magic  lantern,  the  phantasmagoria,  and  numberless 
other  instruments  of  the  same  sort,  most  of  which  will 
be  described  in  the  latter  part  of  this  book. 

There  are  two  sorts  of  microscopes,  the  simple  and 
the  compound ;  the  one  consisting  of  a  single  ctmvex 
lens,  and  the  other  of  several  combinations  of  both 
convex  and  concave  lenses. 

When  speaking  of  convex  lenses,  we  described  the 
properties  of  the  ordinary  magnifying  glass,  or  simple 


142 


THE  WONDERS  OF  Ol'TlCS. 


microscope.  The  uses  of  this  instrument  are  almost  too 
well  known  to  need  description.  It  is  used  by  old 
people,  the  lenses  of  whose  eyes  have  become  flattened 
by  old  age,  by  watchmakers  for  examining  the  minute 
portions  of  their  work,  by  jewellers  for  the  same  pur- 
pose, and  by  most  people  for  examining  maps,  engra- 
vings, and  photographs.  Simple  microscopes  are 
generally  mounted  in  horn,  ivory,  or  metal  handles  for 
convenience'  sake.  Some  simple  microscopes  consist 
of  two  or  more  lenses  mounted  together  in  order  to  in- 
crease the  magnifying  power.  The  student  must  dis- 
tinguish between  several  lenses  mounted  together  in 
this  way,  and  the  true  compound  microscope,  which  is  a 
comparatively  complicated  optical  arrangement,  as  we 
shall  see  presently.  When  two  single  lenses  are  thus 
mounted  together,  the  power  of  the  combination  is  equal 
to  the  powers  of  each  added  together. 

There  is  good  reason  for  supposing  that  the  simple 
microscope  is  a  comparatively  ancient  invention.  Sen- 
eca, who  lived  in  the  first  century,  declares  that  in  his 
time  it  was  well  known  that,  when  writing  was  looked 
at  through  a  globe  full  of  water,  it  appeared  larger  and 
blacker.  In  the  eighth  century  we  find  the  use  of  mag- 
nifying spectacles  for  old  people  common  in  most  coun- 
tries, and  yet  it  was  only  at  the  beginning  of  the  seven- 
teenth century  that  a  true  optical  instrument,  in  the 
form  of  a  telescope,  was  invented.  It  only  needed  the 
placing  of  two  magnifying  glasses  in  a  line  to  discover 
the  principle  of  the  telescope,  but  nearly  a  thousand 
years  elapsed  after  the  first  introduction  of  these  glasses 
before  an  accident  rendered  the  principle  evident. 

In  fiV  37  we  <?ee  the  commonest  form  of  microscope 
in  the  hands  of  an  observer;  and  by  examining  the 
following  figure  and  tracing  out  the  path  of  the  rays, 
we  shall  easily  discover  the  principles  on  which  its 
action  depends. 


OPTICAL  INSTRUMENTS. 


143 


The  object  to  be  looked  at  is  placed  at  a  (fig.  38),  on 
a  piece  of  thin  glass  usually  called  a  slide.    A  small 


Fig.  37.— The  Compound  Microscope. 


converging  lens  placed  at  b  collects  the  rays  proceeding 
from  the  object,  and  transmits  them  as  far  as  c  d,  where 
they  come  under  the  influence  of  a  second  converging 


144 


THE  WONDERS  OF  OPTICS. 


lens  B,  which  causes  them  to  spread  out  still  more  be- 
fore they  reach  the  eye.  Consequently  we  not  only 
see  the  imag<i  of  the  object  magni- 
fied by  the  lens  6,  but  still  more  en- 
larged by  the  action  of  the  lens  B, 
and  appearing  considerably  enlarged 
at  c  D.  The  lens  placed  in  front  of 
the  object  is  called  the  objective  or 
object-glass  ;  that  placed  nearest  the 
eye,  the  eye-piece.  These  names 
apply  equally  to  the  similar  lenses 
used  in  telescopes  and  other  optical 
instruments.  The  instrument  shown 
in  fig.  38  is  the  simplest  possible 
compound  microscope,  and  is  very 
rarely  used.  The  eye-piece  is  gen- 
erally constructed  of  two  lenses,  and 
the  object-glass  of  as  many  as  eight ; 
the  object  in  multiplying  the  lenses 
being,  not  only  to  increase  the  mag- 
nifying power,  but  to  decrease  cer- 
tain defects  inherent  in  all  lenses 
whose  surfaces  are  parts  of  spheres. 

The  amplification  depends  mainly 
upon  the  power  of  the  objective, 
but  different  eye-pieces  are  also  used 
to  increase  the  apparent  size  of  the 
objects  to  be  examined.  Thanks  to 
the  investigations  of  modern  philosophers,  we  are  ena- 
bled to  magnify  objects  to  2,000  times  their  diameter 
with  perfect  distinctness ;  that  is  to  say,  the  surface  of 
the  object  appears  to  occupy  4,000,000  times  its  natu- 
ral extent.  Under  such  a  power  a  hair  would  appear 
about  six  inches  thick,  a  fine  needle  would  look  like  a 
street  post,  and  a  grain  of  sand  like  a  mass  of  rock. 
Although  it  is  possible  to  employ  compound  microscopes 


Fig.  38.— The  Theory  of  the 
Compound  Microscope. 


OPTICAL  INSTRUMENTS. 


145 


of  such  a  high  magnifying  power  in  the  investigation 
of  certain  classes  of  objects,  all  ordinary  preparations 
are  best  seen  under  a  power  of  500  or  600  diameters. 
It  would  be  utterly  impossible  to  give  our  readers  the 
slightest  idea  of  the  benefits  conferred  on  the  human 
race  by  this  marvellous  instrument.  Suffice  it  to  say, 
that  no  naturalist  or  surgeon  ever  attempts  the  most 
simple  investigation  into  the  structure  of  any  body 
without  the  aid  of  the  microscope.  It  has  already 
shown  us  that  a  world  of  creatures  exists  which,  although 
invisible  to  the  eye  of  man,  are  possessed  of  wonderful 
forms,  colour,  and  beauty  of  structure,  and  is  daily 
adding  to  our  knowledge  in  this  direction.  We  can 
hardly  submit  any  substances  to  this  marvellous  instru- 
ment without  discovering  animal  or  vegetable  life  of  the 
most  vivid  character.  A  crop  of  scum  from  the  surface 
of  a  stagnant  pool  is  instantly  seen  to  be  peopled  with 
animal  and  vegetable  life,  when  submitted  to  microscopic 
examination.  At  one  moment  a  rolling  ball  glistening 
like  glass  slowly  revolves  past  our  view ;  then  a  little 
fellow  like  a  piece  of  spiral  spring  screws  his  way  along, 
backing  when  he  meets  with  an  obstacle  ;  or  a  shuttle- 
shaped  vegetable,  apparently  made  of  glass,  with  green 
balls  inside  him,  slowly  works  his  way  from  side  to  side, 
or,  possibly,  a  mad  battledore-shaped  being  dashes  past 
at  an  inconceivable  rate. 

As  it  is  indispensable  that  the  object  should  be  well 
lighted,  a  concave  mirror  is  placed  below  it  to  reflect 
the  rays  of  light  from  a  lamp  or  white  cloud,  through  the 
object  when  it  is  transparent.  When  it  is  opaque,  it  is 
illuminated  by  the  rays  of  light  being  concentrated 
upon  it  by  means  of  a  convex  lens.  The  name  microscope 
appears  by  common  consent  to  be  applied  more  parti- 
cularly to  the  compound  instrument,  the  epithet  of 
magnifier  or  magnifying-glass  being  kept  for  simple 


146 


THE  WONDERS  OF  OPTICS. 


microscopes,  although  they  are  all,  strictly  speaking, 

microscopes. 

In  the  ordinary  compound  microscope,  it  is  only 
possible  for  one  person  to  see  the  object  to  be  examined 
at  once;  for  popular  exhibitions  of  microscopic  objects 
the  reflecting  microscope  has  been  devised,  by  means  of 
which  the  images  of  the  objects  to  be  looked  at  are 
thrown  upon  a  screen.  The  principle  of  this  instrument 
is  the  same  as  that  of  'the  magic  lantern  and  phantas- 
magoria, of  which  we  shall  speak  presently.  Fig.  39 
(see  next  page)  represents  the  photo-electric  microscope, 
so  called  from  the  objects  being  reflected  by  the  electric 
light. 

The  jars  seen  on  the  ground  are  the  cells  of  a  voltaic 
battery,  by  which  the  electricity  is  generated.  The 
luminous  rays  starting  from  the  incandescent  charcoal 
points  are  reflected  through  the  tube  and  its  lenses  by 
the  reflector  placed  at  the  back  of  the  instrument,  and 
are  concentrated  upon  the  object  to  be  magnified.  The 
image  thus  produced  passes  through  a  second  system  of 
converging  lenses,  and  is  projected  upon  the  screen 
magnified  some  millions  of  times  according  to  the  power 
of  the  object-glass  employed. 

"  The  experiments  made  with  the  photo-electric  micro- 
scopes," says  M.  Ganot,  "  are  amongst  the  most  curious 
and  pleasing  to  be  found  in  the  whole  range  of  physical 
science.  With  this  instrument  it  is  possible  to  show  the 
smallest  objects  magnified  almost  indefinitely  to  an  un- 
limited number  of  spectators.  A  human  hair  will  appear 
as  large  as  a  broomstick,  an  ordinary  flea  will  look  the 
size  of  a  sheep,  and  the  tiny  cheese  mite,  as  well  as  the 
smallest  animalcules,  will  be  visible  in  all  their  beauty  of 
form  and  colour  as  clearly  as  if  they  were  seen  with  the 
naked  eye.  One  of  the  most  remarkable  experiments  to 
be  made  with  this  instrument  is  that  which  shows  the 
circulation  of  the  blood.    The  tail  of  a  live  tadpole 


OPTICAL  INSTRUMENTS. 


147 


is  inserted  between  two  plates  of  glass,  or  on  an  instru- 
ment specially  made  for  the  purpose,  and  placed  in  the 
microscope  armed  with  a  somewhat  low  power.  The 


Fig.  39  —Photo  Electric  Microscope. 

spectator  immediately  perceives  upon  the  screen  a  mass 
of  rivers  and  rivulets,  all  flowing  with  the  red  corpuscles 
forming  the  blood  of  the  animal,  and  rushing  through 


148 


THE  WONDERS  OF  OPTICS. 


its  veins  and  arteries  with  inconceivable  rapidity. 
Another  interesting  experiment  consists  in  dissolving  a 
small  quantity  of  sal-ammoniac  in  warm  water,  and 
passing  a  small  portion  of  the  solution  across  a  warm 
glass  slide.  When  placed  in  the  microscope  the  water 
gradually  evaporates,  leaving  behind  a  mass  of  feathery 
crystals,  whose  growth  may  be  watched  atom  by  atom, 
each  crystalline  molecule  grouping  itself  around  the 
others  in  forms  resembling  a  mass  of  fern-leaves." 

The  apparatus  we  have  been  describing  is  sometimes 
illuminated  with  the  rays  of  the  sun,  as  in  the  follow- 
ing figure. 


FlQ.  40. — Solar  Microscope. 

It  is  then  called  the  solar  microscope,  and  exhibits 
objects  with  great  beauty  and  clearness.  The  use  of  the 
sun's  rays,  however,  has,  in  our  own  country  at  least, 
been  entirely  superseded  by  the  electric  and  lime  light. 
The  latter  method  of  illumination,  which  consists  in 
projecting  a  stream  of  oxygen  and  hydrogen  upon  a 
ball  of  lime,  is  cheaper  and  more  certain  than  the  elec- 


OMKJAL  INSTRUMENTS. 


149 


trie  light,  although  the  latter  is  possibly  the  more  bril- 
liant of  the  two.  The  construction  of  the  solar  micro- 
scope differs  but  little  from  the  instrument  already 
described,  and  may  be  readily  understood  from  the 
foregoing  figure.  The  large  mirror  is  placed  outside 
the  window  of  the  room  in  which  the  microscope  stands, 
so  that  the  solar  rays  are  reflected  upon  the  surface  of 
a  series  of  convergent  lenses,  and  from  thence  on  to 
another  mirror,  from  which  it  is  again  reflected  through 
the  microscope.  As  the  position  of  the  sun  is  constantly 
changing,  it  is  necessary  to  connect  the  outside  mirror 
with  a  train  of  clockwork.  It  may  be  mentioned  that 
an  instrument  of  this  kind,  for  reflecting  the  sun's  rays, 
is  called  a  heliostat. 

The  student  will,  no  doubt,  at  once  perceive  that  if 
we  concentrate  the  light  of  the  sun  upon  an  object,  we 
shall  also  concentrate  the  heat,  and  either  melt  or  con- 
sume it.  A  screen  is  therefore  used  in  such  cases, 
which  will  allow  the  light  to  pass  while  holding  back 
the  rays  of  heat.  A  solution  of  alum  is  found  to  answer 
the  purpose  admirably. 


150 


THE  WONDERS  OF  OPTICS. 


CHAPTER  IX. 

THE  TELESCOPES  OF  GALILEO,  GREGORY,  NEWTON, 
HERfeCHEL,  LORD  ROSSE,  AND  FOUCAULT. 

If  history  has  failed  to  furnish  us  with  the  name  of 
the  inventor  of  the  microscope,  we  have  very  exact  in- 
formation as  to  the  first  experimenters  upon  the  powers 
of  the  telescope. 

"  In  he  archives  of  the  Hague,"  says  Arago,  "we 
find  documents,  by  the  aid  of*  which  Van  Swieten  and 
Moll  have  come  to  a  decisive  conclusion  as  to  the  first 
and  true  inventor  of  the  telescope." 

We  read  in  these  documents  that  a  spectacle-maker 
of  Middleburg,  named  John  Lippershey,  addressed  a 
petition  to  the  States-General  on  October  2,  1606,  in 
which  he  asked  leave  to  take  out  a  patent,  which  should 
constitute  him  the  only  maker  of  this  instrument,  or 
which  should  confer  upon  him  an  annual  pension,  on 
the  condition  of  not  manufacturing  them  for  other  na- 
tions. The  petition  qualifies  the  instrument  as  serving 
to  see  distinct  objects,  as  had  already  been  explained 
to  the  members  of  the  States-General. 

On  the  4th  of  October,  1608,  the  States-General 
appointed  a  deputy  from  each  province  to  experiment 
on  the  new  instrument,  which  was  placed  on  a  tower  of 
the  palace  belonging  to  the  Stadtholder.  Huggard 
says  that  the  first  telescopes  experimented  on  were  a 
foot  and  a  half  in  length. 


THE  TELESCOPE. 


151 


On  the  6th  of  October,  the  commission  declared  the 
instrument  of  Lippershey  to  be  useful  to  the  nation,  but 
demanded  that  it  should  be  made  for  two  eyes  instead 
of  one. 

On  the  9th  of  December,  Lippershey,  having  an- 
nounced that  he  had  solved  the  problem,  Van  Dorth, 
Magnus,  and  Van  der  Au  were  ordered  to  verify  the 
fact,  which  they  did  by  making  a  very  favourable  re- 
port on  the  11th  of  the  same  month.  The  binocular 
instrument  was  therefore  found  to  answer. 

In  reading  the  extracts  from  the  archives  of  the 
Hague,  given  by  Moll,  we  may  remark  with  great  plea- 
sure the  promptitude  with  which  the  commissioners  of 
the  States-General  examined  Lippershey's  instruments. 
But  their  satisfaction  soon  gave  way  to  displeasure, 
when  they  found  a  large  number  of  opticians  making 
these  instruments,  and  selling  them  to  foreigners,  like 
so  much  spice  from  the  East  Indies.  Later  on  one  feels 
indignant  at  finding  the  commissioners  of  the  States- 
General  to  be  so  wanting  in  proper  feeling  as  to  decide 
that  the  telescope  must  be  considered  imperfect  until  it 
could  be  used  with  both  eyes,  without  either  winking  or 
seeing  the  reflection  of  the  pupils  in  the  eye-pieces. 
Consequently,  instead  of  being  permitted  to  expend  his 
talent  on  perfecting  the  optical  powers  of  the  single 
telescope,  Lippershey  saw  himself  condemned  to  waste 
his  time  upon  the  double  instrument.  The  States- 
General  finished  by  giving  Lippershey  900  florins ;  but 
they  refused  him  a  patent,  on  the  ground  that  it  was 
already  notorious  that  other  opticians  had  commenced 
the  manufacture  of  similar  instruments. 

Amongst  others  who  were  rivals  of  Lippershey,  we 
must  mention  John  Adrian  Metius,  the  son  of  Adrian 
Metius,  of  Amsterdam,  who  discovered  that  the  nearest 
relation  of  the  circumference  of  a  circle  to  its  diameter 
was  355  to  113.    He  addressed  a  letter  to  the  States- 


152 


THE  WONDERS  OF  OPTICS. 


General  on  the  17th  of  October,  1608,  conceived  in  the 
following  terms : — 

"After  two  years'  labour  and  thought  I  have  succeeded 
in  making  an  instrument,  by  the  aid  of  which  objects 
which  are  too  distant  to  be  visible  by  the  eye,  are  seen 
plainly.  The  one  I  show,  although  constructed  outyof 
bad  materials,  and  simply  as  an  experiment,  is,  in  the 
judgment  of  the  Stadtholder  and  of  several  other  per- 
sons, as  good  as  the  one  lately  presented  to  the  States- 
General  by  a  citizen  of  Middleburg.  I  am  sure  of  im- 
proving it  still  further  in  the  course  of  time,  and  I  beg 
to  ask  for  a  patent  by  which  any  person  who  is  not 
already  in  possession  of  this  invention  will  be  forbidden, 
under  pain  of  a  heavy  fine  and  confiscation,  to  make  or 
sell  similar  instruments  for  twenty-two  years." 

The  States- General  refused  to  grant  the  patent  in 
this  case  also,  but  enjoined  Metius  to  perfect  his  instru- 
ment, reserving  to  themselves  the  power  to  reward  him 
in  the  future  if  they  thought  fit. 

In  Italy,  Galileo  is  generally  supposed  to  have  dis- 
covered independently  the  method  of  making  a  telescope 
on  the  principle  of  the  Dutch  philosophers,  about  the 
beginning  of  1609,  having  received  a  very  imperfect 
account  of  these  instruments  somewhere  about  that 
time.  It  may  be  remarked  that  in  his  letter  to  the 
chiefs  of  the  Venetian  Republic,  giving  an  account  of 
the  properties  of  these  new  instruments,  Galileo  states 
that,  if  necessary,  they  could  be  made  specially  for  the 
use  of  the  navy  and  army  belonging  to  the  state.  But 
secrecy  was  useless,  for  telescopes  were  already  made 
and  sold  in  Holland  at  a  cheap  rate.  Besides,  Galileo 
makes  no  allusion  to  the  labours  of  his  Dutch  predeces- 
sors, either  in  a  prior  letter  handed  down  to  us  by 
Venturi,  or  in  the  decree  of  the  Venetian  Senate,  dated 
August  5,  1609. 

The  Italian  commentators  are  in  error  when  they 


THE  TELESCOPE. 


153 


attribute  the  second  discovery  of  the  telescope  to  the 
knowledge  that  Galileo  possessed  of  the  laws  of  refrac- 
tion, and  that  it  was  by  deductions  therefrom  that  he 
was  enabled  to  construct  his  first  instruments. 

Huyghens  says,  in  his  Treatise  on  Dioptrics,  u  I  will 
unhesitatingly  place  that  man  above  all  mortals,  who, 
by  the  aid  of  his  own  reflections  and  without  the  aid  of 
accident,  first  succeeded  in  constructing  a  telescope." 

"Let  us  see,"  says  Arago,  when  speaking  on  this 
subject,  "  if  Lippershey  and  John  Adrian  Metius  were 
men  of  unparalleled  powers.' ' 

Hieronymus  Saturnus  tells  us  that  an  unknown  man 
of  genius  called  upon  Lippershey,  and  ordered  from  him 
a  number  of  convex  and  concave  lenses.  At  the  time 
agreed  upon  the  man  returned,  and  chose  two,  one  con- 
vex and  the  other  concave,  and,  placing  them  one  before 
his  eye  and  the  other  at  some  distance  from  it,  drew 
them  backwards  and  forwards,  without  giving  any  ex- 
planation of  his  manoeuvres,  paid  the  optician,  and  left 
the  place.  As  soon  as  he  was  gone,  Lippershey  began 
immediately  to  imitate  the  experiments  of  the  stranger, 
and  soon  found  that  distant  objects  were  brought  appa- 
rently nearer,  when  the  lenses  were  placed  in  certain 
positions.  He  next  fastened  them  to  the  ends  of  a  tube, 
and  lost  no  time  in  presenting  the  new  instrument  to 
Prince  Maurice  of  Nassau. 

According  to  another  version,  Lippershey's  children 
were  playing  in  their  father's  shop,  and  were  looking 
through  two  lenses,  one  convex  and  the  other  concave, 
when  they  found  to  their  surprise  that  the  vane  on  the 
clock-tower  of  Middleburg  Church  was  greatly  magni- 
fied and  apparently  brought  nearer.  The  surprise  ex- 
pressed by  the  children  having  awakened  the  attention 
of  Lippershey,  he  tried  the  experiment  of  fixing  the 
lenses  on  a  piece  of  board ;  afterwards  he  tried  it  again 
by  fixing  them  at  the  ends  of  two  pieces  of  tube,  eliding 


154 


THE  WONDERS  OF  OPTICS. 


in  each  other,  and  succeeded  in  making  the  first  tele- 
scope on  record. 

The  principal  documents  from  which  the  above  facts 
touching  Lippershey  have  been  extracted,  are  to  be 
found  in  a  memoir  on  the  subject  by  Olbers,  printed  in 
Schumacher's  Astronomical  Annual  for  1843. 

It  was  said  in  the  time  of  Galileo  that  he  had  in  his 
possession  a  telescope  by  the  aid  of  which  he  could  see 
the  birds  flying  at  Fiesole  from  the  window  of  his  palace 
in  Florence.  This  story  does  not  in  the  least  detract 
from  the  merit  of  the  illustrious  astronomer,  who  not 
only  constructed  a  telescope  for  himself,  but  was  the 
first  to  direct  it  heavenwards,  and  that  too  by  purely 
theoretical  researches ;  for  in  spite  of  all  the  documents 
adduced  above,  there  is  little  or  no  proof  that  he  had 
ever  seen  or  heard  of  the  Dutchman's  telescope.  It  is 
only  right,  therefore,  that  the  instrument  constructed 
on  this  principle  should  be  called  the  Galilean  tele- 
scope, He  afterwards  increased  its  power  from  four  to 
thirty  times,  beyond  which  he  could  not  get  with  the 
means  at  his  command.  With  his  imperfect  instruments 
Galileo  discovered  the  satellites  of  Jupiter,  the  moun- 
tains of  the  moon,  and  the  spots  on  the  sun,  and  earned 
for  himself  the  name  of  Lynceus,  who  according  to  the 
ancients  was  one  of  the  Argonauts,  possessed  of  the 
power  of  seeing  through  a  wall.  Towards  the  end  of 
his  life,  when  the  old  man  was  blind,  and  the  Academy 
of  the  Lincei  treated  his  hypotheses  with  disdain,  he 
would  laugh  sadly  at  the  name  bestowed  on  him,  and 
the  obstinate  Academy.  Fig.  41  (see  next  page) 
shows  the  path  of  the  rays  in  a  Galilean  telescope.  The 
object-glass  o  is  double  convex,  and  the  eye-piece  obi- 
concave.  The  image  is  formed  between  these  lenses, 
and  the  eye  appears  to  see  it  at  that  point.  The  States- 
General  complained  of  being  obliged  to  shut  one  eye 
when  looking  through  a  telescope,  but  in  1671  a  good 


THE  TELESCOPE. 


155 


Capuchin  monk,  whose  name  was  Cherubino,  placed  two 
telescopes  together,  little  thinking  that  the  moderns 


Fig.  41. — The*  Galilean  Telescope. 


would  imitate  him  in  that  very  worldly  instrument,  the 
opera-glass. 

Everybody  has  noticed  that  when  objects  are  close  to 
us  they  appear  larger  than  whe'n  they  are  at  a  distance ; 
it  accordingly  amounts  to  the  same  thing  whether,  in 
speaking  of  the  power  of  telescopes,  we  sa}  they  mag- 
nify twice,  four  times,  or  a  hundred  times,  or  that  they 
are  brought  within  half,  a  quarter,  or  a  hundredth  of 
their  distance.  Thus  there  is  a  telescope  at  Lord  Rosse's 
Observatory,  at  Parsonstown  in  Ireland,  which  is  the 
finest  yet  constructed.  Its  highest  magnifying  power 
is  6,000,  therefore  every  object  we  look  at  with  it  is 
brought  within  the  6,000th  of  its  distance  from  us. 
Looking  at  the  moon,  for  instance,  we  know  that  our 
satellite  is  distant  some  240,000  miles  from  us  ;  we 
have,  therefore,  only  to  divide  that  number  by  6,000  to 
find  that  by  means  of  this  wonderful  instrument  the 
moon  is  brought  within  40  miles  of  the  earth.  This 
statement,  however,  is  not  strictly  true,  for  it  supposes 
the  whole  of  the  apparatus  used  to  be  theoretically  per- 
fect. 

Kepler,  whose  great  name  is  now-a-days  always  asso- 
ciated with  that  of  Galileo,  but  who  during  their  life- 
time wTas  somewhat  his  rival,  substituted  for  the  single 
lens  forming  the  eye-piece  a  combination  consisting  of 
two  convex  lenses,  in  order  to  obtain  a  larger  field  for 


156  THE  WONDERS  OF  OPTICS. 

observation  than  that  given  by  the  single  bi-concave. 
This  combination  is  commonly  known  as  the  astronomi- 
cal eye-piece.  It  reverses  the  object  looked  at,  but 
for  astronomical  purposes  this  defecf  is  of  no  conse- 
quence* 


Fig.  42. — The  Astronomical  Telescope. 

The  instrument  shown  in  the  above  figure  repre- 
sents an  astronomical  telescope  reduced  to  its  simplest 
form. 

Fixed  parallel  to  the  axis  of  the  larger  telescope  is 


THE  TELESCOPE. 


157 


the  finder,  a  small  telescope  of  low  power  and  large 
field,  used  for  finding  celestial  objects  not  easily  visible 
to  the  naked  eye.  It  is  so  arranged,  that  when  the 
object  is  found  and  carried  to  its  centre,  it  is  also  in  the 
centre  of  the  field  of  the  larger  instrument.  The  handle 
and  the  two  toothed  wTheels  serve  to  raise  or  lower  the 
telescope,  which  is  movable  on  the  horizontal  axis, 
which  supports  it  in  front,  so  that  it  may  be  directed  to 
any  part  of  the  heavens  the  observer  may  desire. 

The  following  figure  shows  the  arrangement  of  the 
lenses,  and  the  path  of  the  rays  through  them,  in  tele- 
scopes of  this  form. 


•  1!^  

V 


Fig.  43.— Section  of  an  Astronomical  Telescope. 

The  convex  lens  which  serves  as  an  object-glass,  gives 
at  a  b  a  reversed  image  of  the  star  A  B.  The  small  con- 
vex lens  which  acts  the  part  of  an  eye-piece,  enlarged 
this  reversed  image  without  changing  its  position,  and 
causes  it  to  be  seen  in  the  line  a'  b'.  This  eye-piece  is 
fixed  at  the  extremity  of  a  tube,  which  is  smaller  than 
that  containing  the  object-glas^,  and  slides  easily  back- 
wards and  forwards  from  the  spot  where  the  image  a  b 
is  found.  The  latter  is  an  indispensable  condition,  for 
it  is  rare  to  meet  two  persons  whose  eyes  are  of  the 
same  focus ;  besides,  the  image  a  b  will  fall  at  a  different 


158 


THE  WONDERS  OF  OPTICS. 


spot  for  objects  at  different  distances:  thus,  if  you  are 
looking  at  the  moon,  and  suddenly  turn  the  instrument 
on  to  a  distant  nebula,  you  will  find  that  the  eye-piece 
requires  adjusting.  In  showing  ordinary  observers  an 
object  in  the  telescope,  it  is  well  to  insist  on  their  moving 
the  eye-piece  backwards  and  forwards  until  distinct 
vision  is  obtained,  for  it  often  happens  that  people  will 
say  they  see  an  object  quite  distinctly,  when  it  is  in 
reality  misty,  and  will  generally  refuse  to  allow  the 
focus  to  be  altered.  It  is  very  singular  how  human 
vanity  or  complaisance  will  step  in  when  some  persons 
are  looking  through  a  telescope.  They  seem  to  think 
that  there  is  some  disgrace  or  rudeness  involved  in  their 
not  being  able  to  see  what  their  predecessors  at  the 
instrument  have  seen.  Poor  John  Leech  leaves  us  an 
amusing  instance  of  this  in  a  comic  cut  inserted  in 
one  of  the  early  numbers  of  our  old  friend  Punch,  A 
gentleman  is  endeavouring  to  show  a  lady  a  distant 
steamboat  through  a  telescope,  but  she  has  it  acci- 
dentally pointed  at  two  swans  that  are  sw7imming  on 
the  margin  of  the  lake  below  ;  consequently  when  he 
asks  her  if  she  sees  the  steamer,  she  replies  that 
"  she  sees  it  most  distinctly,  and  there  are  two  of 
them,"  a  pretty  good  proof  that  the  instrument  was 
not  only  pointed  at  the  wrong  object,  but  was  out  of 
focus  as  well. 

In  constructing  a  telescope  similar  to  the  one  de- 
scribed above,  the  object-glass  ought  to  be  of  consider- 
able diameter  and  of  long  focus;  the  eye-piece,  on  the 
contrary,  should  be  comparatively  small  and  of  short 
focus.  A  little  consideration  will  show  the  reason  of 
this.  An  object-glass  of  long  focus  will  form  a  large 
image  at  the  point  a  6,  and  the  eye-piece  of  short  focus 
will  magnify  this  image  more  than  another  lens  of  less 
convexity.  It  is,  however,  on  the  size,  length  of  focus, 
and  perfection  of  workmanship  of  the  objective  that  the 


THE  TELESCOPE. 


159 


excellence  of  the  telescope  depends  ;  large  object-glasses 
are  consequently  rare,  and  are  only  to  be  found  in  ob- 
servatories of  the  first  class.  The  object-glass  of  the 
large  telescope  at  the  Observatory  at  Paris  is  nearly 
fifteen  inches  in  diameter,  and  the  highest  magnifying 
power  capable  of  being  employed  with  it  is  3,000.  The 
Observatory  of  Pulkowa,  near  St.  Petersburg,  possesses 
a  similar  instrument,  and  the  Observatory  at  Chicago, 
United  States,  a  still  larger  one,  measuring  between 
eighteen  and  nineteen  inches  in  diameter.  But  the 
largest  of  all  is  an  objective  in  the  possession  of  Mr. 
Buckingham,  an  amateur  astronomer,  wdio  has  an  ob- 
servatory near  London,  which  is  twenty  inches  in  dia- 
meter, and  twenty-eight  feet  in  focal  length. 

The  eye-pieces  of  astronomical  telescopes  are  of  dif- 
ferent powers,  and  are  changed  according  to  the  class 
of  object  to  be  observed.  Thus,  in  taking  a  general 
view  of  the  moon,  a  low  power  would  be  used.  If  you 
wished  to  examine  any  particular  mountain,  you  would 
raise  the  magnifying  power  by  inserting  a  stronger  eye- 
piece. The  power  used  also  depends  on  the  state  of  the 
atmosphere.  For  instance,  on  warm  evenings,  when  the 
air  is  charged  with  moisture,  the  tremulousness  of  the 
atmosphere  is  so  great,  that  it  is  often  only  possible  to 
use  the  very  lowest  power.  By  combining  four  convex 
lenses  together,  we  obtain  what  is  called  a  terrestrial  or 
erecting  eye-piece,  which  has  the  property  of  re-reversing 
the  image  formed  by  the  objective.  The  eye-pieces  of 
all  telescopes  for  use  on  land  or  at  sea  are  made  on  this 
principle.  The  same  effect  may  be  obtained,  as  we  have 
already  shown  in  fig.  41,  by  using  a  concave  lens,  but 
in  this  the  field  of  view  is  much  diminished. 

Hitherto  we  have  only  spoken  of  refracting  telescopes 
or  those  instruments  provided  with  a  convex  object- 
glass,  to  collect  and  refract  the  rays  of  light  given  off 
by  the  object  we  are  desirous  of  examining;  but  there  is 


160 


THE  WONDERS  OF  OPTICS. 


another  and  very  important  class  of  instruments,  in 
which  the  object-glass  is  replaced  by  a  reflecting  mirror. 
The  first  reflecting  telescope  was  invented  by  Dr.  Gre- 
gory, an  English  philosopher,  about  1650.  It  con- 
sisted  of  a  brass  tube,  at  the  lower  extremity  of  which 
was  fixed  a  concave  mirror  made  of'metal,  and  provided 
with  a  hole  in  its  centre  for  the  insertion  of  the  small 
tube  containing  the  eye-glass.  Towards  the  other 
end  of  the  telescope  was  placed  a  second  and  smaller 
mirror,  which  reflected  the  image  formed  by  the  large 
mirror,  through  the  eye-piece  to  the  eye.  The  follow- 
ing figure  will  show  the  path  of  the  rays  in  the  Grego- 
rian telescope. 


Fio.  44.— Section  of  the  Gregorian  Telescope. 

The  rays  A  b,  proceeding  from  the  object  at  which 
the  instrument  is  pointed,  are  first  reflected  from  the 
surface  of  the  principal  mirror  M  M  on  to  the  small 
mirror  ra,  whence  they  proceed  to  form  a  magnified 
image  at  a  b,  which  is  then  again  enlarged  by  the  eye- 
piece appearing  to  the  eye  as  if  placed  at  a'  b'.  The 
focus  in  the  Gregorian  is  altered,  not  by  sliding  the 
eye-piece  backwards  and  forwards  but  by  moving  the 
mirror  m,  which  is  provided  with  a  long  screw,  to  which 
is  attached  a  handle.    At  first  sight  a  reflecting  tele- 


THE  TELESCOPE.  161 

scope  has  the  appearance  of  a  very  stumpy-looking  re- 
fracting instrument,  but  one  instant's  examination  will 
show  the  observer  that  the  usual  object-glass  is  absent 
at  the  end  of  the  tube.  In  fig.  45  we  have  a  Gregorian 
telescope,  mounted  on  a  tripod  stand. 


Fig.  45. — Gregorian  Telescope. 

Whilst  experimenting  on  the  Gregorian  telescope, 
Newton  made  certain  improvements  in  its  construction, 
which  we  shall  proceed  to  describe.    A  glance  at  fig. 


162 


THE  WONDERS  OF  OPTICS. 


46*  will  show  that  the  path  of  the  rays  is  much  more 
simple  than  in  the  instrument  we  have  just  noticed. 


w  ■ — — f 

iM  ______ 

Km 

B 

Fig.  46. — Section  of  a  Newtonian  Telescope 


The  rays  of  light  A  b  are  first  reflected  from  the 
concave  mirror  M  on  to  the  surface  of  the  small  plane 
mirror  m,  which  is  placed  at  an  angle  of  45°,  and  re- 
flects them  as  far  as  the  point  Af  b',  where  they  form 
the  image  to  be  magnified  by  the  eye-glass.  It  is 
therefore  at  the  side  of  the  instrument,  and  not  at  the 
end,  as  hitherto,  that  the  observer  is  placed,  and  at 
right  angles  to  the  path  of  the  rays.  Observers  look- 
ing at  an  object  through  a  Newtonian  telescope  for  the 
first  time  are  generally  sufficiently  astonished  to  find 
that  there  is  really  no  difficulty  after  all  in  seeing  round 
a  corner.  We  shall  presently  return  to  the  subject  of 
Newtonian  telescopes,  which  were  abandoned  by  astro- 
nomers for  many  years,  until  they  were  brought  into 
use  again  by  M.  Foucault,  a  distinguished  French  phi- 
losopher. 

Towards  the  end  of  the  last  century  Sir  William 
Herschel  invented  and  constructed  the  reflecting  tele- 
scope which  bears  his  name.  His  great  object  was  to 
avoid  the  loss  of  light  consequent  on  the  double  reflec- 
tion which  took  place  in  all  instruments  constructed  up 


THE  TELESCOPE. 


163 


to  that  time,  and  he  succeeded  at  last  in  making  a  tel- 
escope in  which  the  observer  looked  directly  through  the 
eye-piece  at  the  image  formed  by  the  mirror,  which  was 
inclined  in  such  a  manner  that  the  rays  were  reflected 
to  the  lower  edge  of  the  open  end  of  the  tube.  In 
using  this  kind  of  telescope  the  observer  is  placed  with 
his  back  to  the  object  he  wishes  to  examine,  a  position 
that  is  even  more  astonishing  to  those  unaccustomed  to 
the  use  of  a  Herschellian  telescope  than  the  one  assumed 
when  employing  an  instrument  of  the  Newtonian  con- 
struction. This  position  has  the  defect  of  causing  a 
small  portion  of  the  rays  proceeding  from  the  object  to 
be  intercepted  by  the  head  of  the  observer,  but  the 
amount  of  light  lost  is  so  small  in  comparison  to  the 
size  of  the  mirror  that  in  practice  it  amounts  to  nothing. 

The  dimensions  of  the  telescope  constructed  by  Her- 
schel  were  enormous  for  that  day.  It  measured*40  feet 
long,  and  the  mirror  was  4  feet  in  diameter.  It  was 
supported  by  a  complicated  system  of  scaffolding,  pul- 
leys, and  cords,  and  was  capable  of  magnifying  an  object 
6,000  times.  It  was  by  means  of  this  splendid  instru- 
ment that  Sir  William  Herschel  made  those  wonderful 
discoveries  in  astronomy  which  are  inseparably  associa- 
ted with  his  name.  With  it  he  discovered  the  planet 
Uranus,  many  of  the  double  stars,  and  a  large  number 
of  nebulae,  which  up  to  that  time  were  unknown.  His 
&on,  Sir  John  Herschel,  inherits  his  father's  talents  as 
an  astronomer,  and  has  enriched  science  with  number- 
less observations  and  discoveries  of  the  greatest  impor- 
tance made  with  this  fine  instrument.  Fig.  47  shows 
the  construction  of  the  Herschellian  telescope,  and  the 
path  of  the  rays  may  be  easily  followed  by  the  student 
without  any  help  from  us. 

The  vulgar,  ever  prone  to  make  mountains  out  of 
molehills,  magnified  the  power  of  Sir  William  HerschrVs 
telescope  beyond  all  bounds.    Stories  were  circular  ^d 


164 


THE  WONDERS  OF  OPTICS. 


about  his  having  given  a  dinner  in  the  interior  of  the 
tube  to  a  select  party  of  friends,  but  as  the  diameter 
of  the  telescope  was  only  a  little  more  than  4  feet,  the 
entertainment,  to  say  the  least  of  it,  would  have  proved 


f  ^  B' 

Fig.  47.— The  Herschellian  Telescope. 

somewhat  inconvenient  to  the  guests.  Another  story, 
which  was  credited  by  great  numbers  of  people,  was 
that  he  had  discovered  inhabitants  in  the  moon,  but  that 
he  hesitated  to  make  the  matter  public  for  fear  he  should 
be  prosecuted  for  spreading  atheistical  notions.  In  fact, 
the  tales  told  of  Sir  William  Herschel's  telescope  were 
endless,  and  caused  the  astronomer  great  inconvenience 
by  attracting  crowds  of  idle  people  to  the  neighbour- 
hood of  Slough,  where  he  vainly  endeavoured  to  carry 
on  his  investigations  in  peace  and  quietness.  It  was  in 
vain  that  these  silly  assertions  were  disproved  again 
and  again.  Having  once  believed  them,  people  were 
slow  to  reject  them,  and  the  story  of  the  dinner  was 
told  over  and  over  again  for  many  years. 

The  instrument  above  described  is  one  of  those  known 
as  front  view  tele-scopes,  on  account  of  the  image  of  the 
star  being  reflected  from  the  surface  of  the  mirror, 
which  was  placed  obliquely  at  the  bottom  of  the  tube 
in  front  of  the  observer,  who  examined  it  by  means  of 
the  eye-piece  without  any  other  reflection  taking  place. 


THE  TELESCOPE. 


165 


thereby  effecting  a  saving  of  light,  which  fully  compen- 
sated for  any  loss  caused  by  the  mirrior  being  placed 
askew.  The  concave  mirror  made  by  Herschel  alone 
weighed  a  ton,  to  say  nothing  of  the  enormous  tube  and 
its  fittings.  Herschel  had  consequently  to  invent  a 
special  apparatus  for  holding  and  moving  this  gigantic 
instrument.  The  moving  gear  consisted  of  a  mass  of 
beams,  pulleys  and  cords,  reminding  one  more  of  the 
rigging  of  a  ship  than  of  a  philosophical  instrument. 
The  apparatus  for  moving  the  telescope  appeared  so 
complicated  to  the  casual  observer,  although  in  reality 
it  was  very  simple,  that  it  doubtless  contributed  in  no 
small  degree  to  the  propagation  of  the  fanciful  stories 
we  have  already  spoken  of. 

The  performances  of  this  splendid  instrument  hardly 
came  up  to  the  expectations  of  those  who  saw  it  in  pro- 
gress. Herschel,  it  is  true,  was  enabled  by  its  means 
to  use  a  power  of  from  3  to  6,000,  but  he  could  only 
use  these  amplifications  on  a  few  objects — the  planets, 
for  instance,  giving  so  little  light  under  a  high  power 
as  to  become  indistinct  and  misty.  In  1802  Baron  von 
Zach,  in  his  Monthly  Astronomical  Compendium,  went 
so  far  as  to  say  that  this  colossal  instrument  was  not  of 
the  slightest  utility,  that  no  discovery  had  ever  been 
made  with  it,  and  that  it  ought  to  be  considered  merely 
as  an  optical  curiosity.  Subsequent  events,  however, 
proved  very  conclusively  that  Baron  von  Zach  was  ut- 
terly wrong  in  his  statements  and  prophecies. 

The  telescope  constructed  by  Herschel,  although 
very  wonderful  for  the  day  in  which  it  was  made,  has 
long  since  been  eclipsed  by  that  belonging  to  Lord  Rosse, 
and  erected  by  his  late  father  at  Birr  Castle,  near 
Parsonstown  in  Ireland.  It  is  superior  to  Herschers 
instrument  both  in  point  of  size,  and  workmanship. 
The  late  Lord  Rosse,  not  fearing  that  his  dignity 
would  be  compromised  by  such  an  act,  went  boldly  to 


166 


THE  WONDERS  OF  OPTICS. 


work,  and  learned  to  polish  mirrors  like  an  ordinary 
workman,  the  consequence  of  which  was  that  he  could 
bestow  unusual  pains  upon  the  finishing  of  the  speculum. 
His  Lordship  not  only  learnt  the  mere  handicraft  of 
speculum  polishing,  but  went  deeply  into  the  engineering 
difficulties  of  the  operation,  and  succeeded  in  inventing 
many  improvements  for  diminishing  labour  and  ren- 
dering the  form  of  the  surface  more  perfect.  The 
specula  ground  and  polished  under  Lord  Rosse's  method 
are  almost  entirely  free  from  what  is  called  spherical 
aberration, — that  is  to  say,  all  rays  proceeding  from 
a  single  point  of  light,  such  as  a  star,  are  collected  into 
a  single  point  instead  of  being  scattered  in  a  round 
mass.  This  freedom  from  spherical  aberration  is  of 
course  necessary  to  produce  perfectly  distinct  images. 
In  his  Life  of  Newton  Sir  David  Brewster  calls  it  one 
of  the  most  marvellous  combinations  of  art  and  science 
yet  seen  in  the  world. 

The  tube  of  Lord  Rosse's  instrument  is  55  feet  long, 
and  weighs  6J  tons.  In  form  it  may  be  compared  to 
the  chimney  of  a  steamboat  of  enormous  size.  At  one 
end  it  terminates  in  a  kind  of  square  box,  within  which 
is  contained  the  mirror,  whose  diameter  is  6  feet,  and 
which  weighs  nearly  4  tons.  The  weight  of  the  whole 
apparatus  is  consequently  nearly  10J  tons,  or  four 
times  as  much  as  Herschel's.  It  is  erected  on  an 
oblong  mass  of  masonry,  75  feet  in  length  from  north 
to  south,  between  two  solid  walls  nearly  50  feet  high, 
which  serve  as  supports  for  the  mechanism  intended  to 
move  this  enormous  tube  in  all  directions.  To  the 
walls  are  also  fixed  movable  staircases  with  platforms 
that  can  be  brought  up  to  the  eye-piece  with  the  great- 
est facility,  no  matter  in  what  position  the  telescope 
may  be  placed.  This  noble  instrument  has  penetrated 
space  to  a  distance  perfectly  unattempted  before  its  ex- 
istence, and  has  resolved  numerous  nebulae  into  masses 


THE  TELESCOPE. 


167 


of  stars  that  until  then  were  supposed  to  be  mere 
clouds  of  luminous  matter.  The  exact  forms  of  other 
nebulae  have  also  been  accurately  determined  by  this 
telescope,  which  fully  deserves  the  glowing  eulogium 
passed  upon  it  by  the  Duke  of  Argyle  in  his  presi- 
dential address  at  the  meeting  of  the  British  Association 
at  Glasgow,  in  1855.  "  This  instrument/'  said  his 
Grace,  "  in  extending  the  range  of  astronomical  science 
as  it  has  done,  has  been  the  means  of  throwi  g  certain 
doubts  upon  the  laws  that  govern  the  motions  of  the 
heavenly  bodies,  and  render  it  possible  that  certain  of 
the  far-distant  nebulae  are  regulated  in  their  movements 
by  other  laws  than  those  to  which  the  members  of  our 
own  system  are  subjected.,, 

The  clearness  with  which  this  telescope  exhibits  every 
object  within  its  range  is  so  great  that  the  most  distant 
nebulae  are  seen  with  as  great  distinctness  as  the  nearest 
planet.  On  directing  it  towards  the  moon,  which  is 
only  distant  from  us  about  24^,000  miles,  the  surface 
of  our  satellite  may  be  explored  with  a  facility  almost 
as  great  as  that  with  which  we  examine  the  details  of  a 
landscape  with  an  ordinary  telescope. 

Maedler,  a  German  astronomer,  who  has  measured 
nearly  every  mountain  and  valley  on  the  moon's  surface 
with  the  greatest  exactitude,  stated  some  years  before 
Lord  Rosse's  telescope  was  perfected  that  if  a  monu- 
ment as  large  as  one  of  the  Pyramids  existed  on  the 
surface  of  the  moon  it  could  have  been  readily  distin- 
guished by  the  instruments  then  in  use.  With  Lord 
Rosse's  telescope  we  can  see  the  surface  of  our  satellite 
so  much  enlarged  that  a  space  220  feet  square  could  be 
readily  perceived  by  a  good  observer.  This  enormous 
eye,  measuring  6  feet  in  diameter,  would  hardly  show 
us  a  lunar  elephant ;  but  it  is  certain  that  if  a  troop  of 
buffaloes,  or  animals  analogous  to  them,  crossed  the 
field  of  vision,  they  would  undoubtedly  be  perceptible. 


168 


THE  WONDERS  OF  OPTICS. 


Masses  of  troops  marching  backwards  and  forwards 
would  also  be  plainly  visible,  and  we  may  assert  with 
something  like  absolute  certainty  that  there  are  neither 
towns  nor  villages  in  the  moon,  nor  any  buildings  as 
large  as  St.  Paul's  of  London  or  the  colossal  railway 
stations  of  that  metropolis. 

This  telescope,  as  we  have  said  before,  is  the  largest 
hitherto  constructed,  and  cost  its  noble  constructor 
more  than  25,000/.  It  must  also  be  recollected  that  it 
was  not  a  mere  scientific  toy  belonging  to  an  amateur 
philosopher,  but  a  real  working  instrument  in  the 
possession  of  a4rue  man  of  science,  who  did  work  with 
it  that  will  render  his  name  famous  while  civilization 
lasts.  The  present  Lord  Rosse  seems  worthy  in  every 
way  of  his  father's  great  name,  and  has  already  en- 
riched astronomical  science  with  numerous  valuable 
observations. 

We  shall  finish  this  chapter  by  a  description  of  the 
Newtonian  telescope  constructed  by  M.  Leon  Foucault. 
The  mirror,  instead  of  being  made  of  speculum  metal, 
which  is  an  alloy  of  tin  and  copper,  is  made  of  glass 
from  the  famous  manufactory  of  St.  Gobain.  The  first 
rough  grinding  having  been  finished,  it  passed  into  the 
workshops  of  M.  Secretan,  the  optician  to  the  Paris 
Observatory,  to  receive  its  final  polish  and  finishing 
touches  from  the  hand  of  M.  Foucault  himself,  the  most 
careful  optical  tests  being  applied  to  it  before  the  com- 
mencement of  each  operation. 

The  glass  mirror-  having  reached  the  degree  of  per- 
fection desired,  was  then  silvered  on  its  concave  surface 
by  being  plunged  into  a  bath  of  nitrate  of  silver, 
dissolved  in  water,  and  mixed  with  certain  proportions 
of  gum  galbanum,  nitrate  of  ammonia,  and  oil  of  cloves. 
Half  an  hour  in  this  bath  was  sufficient  for  the  depo- 
sition of  a  film  of  silver  of  sufficient  thickness  to  bear 
polishing.     When  finished,  the  mirror  was  found  to 


THE  TELESCOPE.  169 

reflect  92  per  cent,  of  the  light  incident  on  its  surface, 
the  loss  in  the  case  of  achromatic  object-glasses  and 
metal  specula  being  20  and  35  per  cent,  respectively. 


Fig.  48.— Foucault's  Large  Telescope. 

The  substitution  of  a  parabolic  glass  mirror  for  the 
ordinary  metal  speculum  offers  the  triple  advantage  of 
greater   lightness,  increased   distinctness,  and  more 


170 


THE  WONDERS  OF  OPTICS. 


brilliant  images.  Fig.  48  represents  the  large  sil- 
vered glass  teiescope  eoastructed  under  M.  Foucault's 
direction  for  the  observatory  at  Marseilles.  It  measures 
32  inches  in  diameter,  and  has  a  focal  length  of  a  little 
more  than  16  feet,  and  is  put  in  motion  by  clockwork  of 
a  very  perfect  description,  so  that  when  once  pointed  at 
a  star  or  planet  it  follows  the  object,  which  would  other- 
wise disappear  on  account  of  the  rotation  of  the  earth. 
The  path  taken  by  the  rays  is  precisely  the  same  as  in 
]S  ewton's  telescope,  the  eye-piece  being  placed  at  the  side 
of  the  tube,  which  is  provided  with  a  movable  platform 
and  staircase  for  the  observer. 

The  optician  to  whose  talent  in  his  art  this  fine  in- 
strument is  due,  has  recently  executed  several  small 
telescopes  upon  the  same  model,  at  such  a  price  as  to 
bring  them  within  the  reach  of  amateurs  with  slender 
purses.  The  principal  part  of  these  telescopes,  one  of 
which  is  represented  in  fig.  49,  (see  next  page),  is  the 
mirror,  which  is  about  4  inches  in  diameter,  and  24 
inches'  focal  length.  The  body,  which  is  cylindrical,  is 
made  of  brass,  and  revolves  on  two  pivots  placed  hori- 
zontally at  about  one-third  of  its  length  from  the  bottom. 
The  bearings  on  which  the  pivots  move  consist  of  two 
upright  standards  of  metal,  which  are  connected  at  the 
bottom,  and  revolve  on  a  pin  in  the  middle  of  the  plate 
of  the  tripod  stand.  They  are  made  of  such  a  height 
that  the  lower  portion  of  the  instrument  may  pass  be- 
tween them,  when  it  is  necessary  to  observe  objects  in 
the  zenith.  By  the  turn  of  a  screw  the  whole  of  the 
upper  portion  of  the  instrument  may  be  dismounted  and 
fixed  on  a  lower  standard,  so  that  the  observer  may  work 
sitting  down  if  necessary.  The  body  of  the  telescope 
is  provided  with  a  finder.  One  of  the  great  advantages 
of  this  form  of  instrument  is  that  it  can  be  used  for 
observations  on  the  zenith  without  giving  the  observer 
those  unpleasant  cricks  in  the  neck  so  inseparable  from 


THE  TELESCOPE. 


171 


che  use  of  ordinary  telescopes  in  a  nearly  upright  con 
dition.    The  mirror  will  bear  a  power  of  220  diameters, 


Fig.  49. — Foucault's  Small  Telescope. 

and  shows  the  mountains  of  the  moon,  the  phases  of 
Mercury  and  Venus,  Saturn  and  his  ring,  Jupiter  and 


172 


THE  "WONDERS  OF  OPTICS. 


his  satellites,  and  a  large  number  of  double  stars  and 
nebulae.  It  is  provided  with  a  set  of  eye-pieces,  so  that 
any  power  almost  from  50  to  220  diameters  may  be 
used  at  will.  The  figure  on  the  opposite  page  will  give 
the  amateur  a  good  idea  of  the  form  and  size  of  this 
instrument. 


THB  TELESCOPE* 


173 


PART  III. 

NATURAL  MAGIC. 


CHAPTER  L 

THE  MAGIC  LANTERN. 

The  illusions  of  which  we  have  spoken  in  the  first 
part  of  this  work  depended  principally  on  the  nature  of 
man's  vision,  who,  we  found,  was  the  constant  and  heed- 
less victim  of  his  own  powers  of  sight.  We  shall  now 
examine  a  series  of  illusions  that  are  still  more  extraor- 
dinary, but  which  have  nothing  to  do  with  the  decep- 
tions practised  on  us  by  our  visual  organs.  Instead  of 
being  deceived  by  ourselves,  we  shall  find  that  we  are 
led  astray  by  others  whose  knowledge  of  the  laws  of 
optics  is  greater  than  our  own,  enabling  them  to  con- 
struct instruments  capable  of  amusing  us  or  imposing 
on  us,  according  to  our  ignorance  of  natural  laws.  Let 
us  hope,  however,  that  the  science  of  optics  has  now 
become  so  familiar  to  most  educated  people,  that  no  such 
thing  as  a  real  imposition  can  take  place,  although  at 
the  present  day  there  are  so  many  exhibitions  of  the 
marvellous  that  ordinary  observers  have  the  greatest 


174 


TTIE  W0NDER8  OF  UllTCS. 


difficulty  in  accounting  for  them.  In  former  ages,  wher 
the  knowledge  of  science  was  confined  to  a  certain  class 
the  commonest  optical  facts  of  the  present  day  were 
taken  advantage  of  to  delude  the  ignorant.  The  de- 
ceptions practised  by  the  ancient  priests  of  Egypt, 
Greece,  and  Rome  were  undoubtedly  many  of  them  of 
this  description.  It  is  a  well  known  fact  that  both  plane 
and  concave  metallic  mirrors  were  used  by  the  ancients, 
and  a  passage  in  Pliny  gives  an  account  of  certain  glass 
mirrors  that  were  made  at  Sidon.  Aulus  Gellius,  quo- 
ting Varro,  speaks  of  the  reflecting  properties  of  hol- 
low mirrors,  and  we  shall  see,  as  we  go  on,  what  a  num- 
ber of  illusions  may  be  practised  by  means  of  a  series 
of  plane  mirrors  arranged  in  a  particular  way.  But  we 
will  first  devote  a  short  time  to  the  curious  historical 
facts  connected  with  the  principle  of  the  magic  lantern 
which  took  place  long  before  the  modern  invention  of 
this  instrument  by  Father  Kircher. 

Brewster  says,  when  treating  of  this  subject,  that 
there  can  be  little  doubt  that  the  concave  mirror  was 
the  principal  instrument  used  in  connexion  with  the 
pretended  apparitions  of  the  gods  and  goddesses  in  the 
ancient  temples.  In  the  meagre  history  of  these 
apparitions  that  has  come  down  to  us,  we  can  easily 
perceive  the  traces  of  an  optical  illusion.  In  the 
ancient  temple  of  Hercules  at  Tyre,  there  existed  a 
certain  seat  made  of  consecrated  stone,  out  of  which 
the  gods  rose,  apparently  at  the  will  of  the  priests. 
iEsculapius  appeared  frequently  to  his  worshippers  in 
his  temple  in  Tarsus,  and  the  temple  of  Eugenium  was 
famous  for  the  number  of  gods  and  goddesses  which 
were  constantly  visiting  its  sacred  precincts.  Iamblicus 
tells  us  that  the  priests  showed  the  gods  to  the  people 
in  the  midst  of  smoke ;  and  when  the  great  magician 
Marinus  terrified  his  auditory  by  suddenly  showing 
them  the  statue  of  Hercules  in  the  midst  of  a  cloud  of 


THE  MAGIC  LANTERN. 


175 


incense,  it  was  undoubtedly  a  woman  who  performed 
the  part,  dressed  up  in  man's  robes  for  the  occasion. 

The  character  of  these  spectacles  in  the  ancient  tem- 
ples is  admirably  described  by  Damasius,  and  there  is 
no  difficulty  in  seeing  that  optical  illusions  were  the 
means  employed  to  delude  the  audience.  He  describes 
the  apparition  on  the  wall  of  a  large  spot  of  white, 
which  at  first  appeared  at  a  distance,  but  gradually 
came  nearer  and  nearer  until  at  last  it  assumed  the 
form  of  a  divine  or  supernatural  being,  of  severe  yet 
mild  aspect  and  of  great  personal  beauty.  This  being 
the  Alexandrians  immediately  honoured  as  Osiris  or 
Adonis. 

Amongst  more  modern  examples  of  this  illusion  may 
be  mentioned  that  of  the  Emperor  Basil  of  Macedonia. 
Inconsolable  at  the  loss  of  his  son,  this  potentate  had 
recourse  to  the  prayers  of  the  Pontiff  Theodore  Lanta- 
baren,  who  was  celebrated  for  his  power  of  working 
miracles.  The  conjurer  showed  the  Emperor  the  image 
of  his  dead  son  magnificently  attired  and  mounted  on  a 
splendid  war-horse.  The  young  man  dismounted,  and, 
going  up  to  his  father,  threw  himself  into  his  arms  and 
disappeared.  Salvertius,  in  speaking  of  this  story, 
observes  judiciously,  that  the  deception  could  only  take 
place  through  the  agency  of  some  person  who  closely 
resembled  the  Emperor's  son,  and  that  the  trick  would 
have  been  easily  discovered  when  the  person  embraced 
the  Emperor.  A  better  explanation  of  the  affair  is, 
however,  afforded  by  supposing  that  the  Emperor  saw 
an  aerial  image  of  a  person  resembling  his  son,  and 
that  when  he  rushed  forward  to  embrace  him  it  dis- 
appeared. 

The  accounts  of  the  operations  of  the  ancient  magi- 
cians are  too  meagre  to  give  us  any  idea  of  the  splen- 
dour of  some  of  these  ancient  ceremonies.  A  system 
of  deception  such  as  this,  employed  as  a  means  of 


176 


THE  WONDERS  OF  OPTICS. 


government,  must  have  brought  into  requisition  not 
only  the  talents  of  all  the  learned  men  of  the  day,  but 
a  crowd  of  accessories  calculated  to  astonish  and  con- 
found the  judgment,  fascinate  the  senses,  and  facilitate 
imposture. 

An  account  of  an  instance  of  modern  necromancy 
has  been  left  us  by  Benvenuto  Cellini,  who  played  a 
prominent  part  in  a  case  of  this  sort. 

He  accidentally  made  the  acquaintance  of  a  Sicilian 
priest,  a  man  of  great  genius  and  acquirements,  and 
well  versed  in  Greek  and  Latin  classical  lore.  One  day 
the  conversation  turned  on  necromancy,  and  the  great 
goldsmith  told  him  that  he  had  the  greatest  desire  to 
know  something  about  this  wonderful  art,  and  that  he 
had  felt  all  his  life  a  great  curiosity  to  penetrate  its 
mysteries. 

The  priest  replied,  that  a  man  ought  to  have  a  very 
resolute  and  fearless  character  to  study  this  art ;  but 
Benvenuto  answered  he  had  both  resolution  and  courage. 
The  priest  went  on  to  say,  that  if  he  had  the  heart  to 
try,  he  would  be  the  means  of  obtaining  the  fulfilment 
of  his  wishes.  They  consequently  agreed  upon  a  plan 
of  necromantic  study.  One  evening,  Benvenuto  invited 
one  of  his  companions,  Vincenzio  Bomoli,  to  take  part 
in  some  experiments  that  were  to  be  made  amongst  the 
ruins  of  the  Coliseum.  They  there  met  the  Sicilian 
priest,  who  after  the  manner  of  the  ancients  began  to 
describe  a  number  of  circles  in  the  air  in  the  most  im- 
posing manner.  He  had  brought  with  him  various  gums 
and  perfumes,  and  had  made  a  fire,  into  which  his 
assistant  necromancer  was  to  throw  them  at  the  proper 
time.  He  commenced  his  conjurations,  the  ceremony 
continuing  about  an  hour,  when  there  appeared  legions 
of  demons,  in  such  numbers  that  the  whole  of  the  ruins 
seemed  filled  with  them.  Benvenuto  was  nearly  faint- 
iag  with  the  perfumes,  when  the  priest  roused  him  by 


THE  MAGIC  LANTERN. 


177 


telling  him  to  ask  for  something.  He  replied,  that 
he  wished  to  be  transported  to  the  side  of  his  Sicilian 
mistress ;  but  the  demons  were  evidently  unpropitious, 
for  nothing  came  of  it.  His  instructor,  however,  told 
him  that  they  must  repeat  their  experiments  a  second 
time,  and  that  Benvenuto  must  bring  with  him  a  child 
that  had  never  committed  sin.  The  next  time  Benve- 
nuto took  with  him  a  boy  of  twelve  years  old  wThom  he 
had  in  his  service,  and  his  friends  Romoli  and  Guddi. 
When  they  arrived  at  the  place  of  meeting,  they  found 
the  priest  had  made  the  same  preparations  as  before. 
This  time,  however,  he  used  more  powerful  conjurations, 
calling  on  a  number  of  demons  by  their  names,  in  He- 
brew, Greek,  and  Latin ;  so  that  the  ruin  was  filled 
with  a  still  greater  mass  of  them  than  on  the  other  oc- 
casion. Tbe  fire  and  perfumes  were  put  under  the 
charge  of  Guddi  and  Romoli,  and  he  gave  Benvenuto  a 
magic  picture  to  hold  in  a  certain  direction,  the  boy 
being  placed  underneath  it.  The  priest  told  him  again 
*;o  wish  to  be  in  the  company  of  his  lady  love,  but  on  his 
expressing  the  wish,  the  magician  told  him  that  the  de- 
mons still  refused  to  do  his  bidding  in  this  way,  but  that 
he  should  visit  her  once  more  in  a  month's  time.  The 
poor  boy  underneath  the  magic  picture  was  seized  with 
a  terrible  fright,  and  exclaimed,  that  he  saw  millions  of 
ferocious  spirits  and  four  giants,  all  endeavouring  to 
break  through  the  magic  circle  the  priest  had  formed. 
All  there  were  evidently  in  a  most  abject  state  of  terror, 
and  remained  in  the  place  until  the  church  bells  began 
to  ring  for  morning  prayers,  when  they  returned  home, 
the  boy  declaring  that  two  of  the  demons  preceded  them, 
dancing  and  gambolling  before  them,  and  sometimes 
running  along  the  housetops. 

The  priest  then  advised  him  to  try  another  spiritual 
seance,  and  endeavour  to  induce  the  demons  to  point 
out  sundry  pots  of  buried  gold,  so  that  they  all  might 


178 


THE  WONDERS  OF  OPTICS. 


become  rich,  but  it  does  not  appear  that  the  priest's 
advice  was  followed. 

It  is  impossible  to  read  the  foregoing  description  of 
what  happened,  without  being  convinced  that  the  whole 
affair  was  an  optical  illusion,  and  not  the  mere  result  of 
the  imagination  of  those  who  took  part  in  it.  The  smoke 
was  evidently  caused  in  order  to  afford  a  field  for  the 
exhibition  of  painted  images  reflected  by  concave  mir- 
rors, and  the  circle  was  formed  in  order  that  those 
within  it  might  be  within  range  of  the  images  formed  on 
the  smoke.  The  mirrors  reflecting  the  images  of  the 
demons  had  undoubtedly  already  been  arranged  so  that 
they  would  fall  just  above  the  fire,  and  become  visible 
when  the  gums  began  to  burn  with  a  smoky  flame.  The 
perfumes  were  simply  to  help  to  stupify  the  spectators, 
and  aid  in  working  on  their  imaginations  for  those  oc- 
currences which  were  beyond  the  reach  of  optics,  for  the 
poor  unfortunate  boy  saw  things  that  his  companions 
did  not,  even  to  a  couple  of  demons  dancing  through 
the  streets  in  broad  djaylight.  In  fact,  it  is  somewhat 
difficult  to  draw  the  line  between  reality  and  imagina- 
tion in  this  case.  No  doubt  the  story  is  considerably 
exaggerated  by  Cellini,  who  was  a  fervid  Italian,  and 
prone  to  believe  in  wonders,  as  is  instanced  by  his  wish 
to  study  the  black  art.  The  priest,  too,  whom  he  de- 
scribes as  a  man  of  genius,  no  doubt  had  a  great  influ- 
ence over  the  famous  artist,  and  made  him  see  a  great 
deal  more  than  was  really  there. 

The  introduction  of  the  magic  lantern  provided  the 
magicians  of  the  seventeenth  century  with  a  very  pow- 
erful instrument  with  which  to  continue  their  deceptions. 
The  use  of  the  concave  mirror,  which  does  not  appear  to 
have  had  any  accessories  worth  speaking  of,  required  a 
separate  apartment,  or  at  least  a  hiding-place  of  some 
sort  that  was  difficult  to  discover  under  ordinary  circum- 
stances ;  but  the  magic  lantern,  inclosing  as  it  did  the 


THE  MAGIC  LANTERN. 


179 


lamp,  the  optical  apparatus,  and  the  figures  in  a  com- 
paratively small  spa.ce,  was  p  rticularly  appropriate  to 
the  wants  of  the  Homes  and  Davenports  of  the  day,  who 
until  then  had  never  possessed  anything  so  convenient 
and  portable. 

The  magic  lantern  shown  in  figures  50  and  51  con- 
sists of  a  dark  box,  containing  a  lamp  and  a  concave 
metallic  mirror,  constructed  in  such  a  way  that  the 


Fig.  50. — Section  of  the  Magic  Lantern. 

whole  of  the  rays  proceeding  from  the  lamp  are  reflected 
through  the  aperture  holding  the  optical  portion  of  the 
apparatus.  In  front  of  the  box  is  fixed  a  double  tube 
CD,  one-half  of  which  (d)  slides  in  the  other.  A  large 
plano-convex  lens  c  is  fixed  at  the  inner  extremity  of  the 
double  tube,  and  a  small  one  at  its  outer  end.  To  the 
fixed  tube  c  E  is  attached  a  groove  b  6,  which  serves  to 
hold  the  painted  glass.  These  glasses,  or  slides  as  they 
are  generally  called,  are  painted  with  strong  transpa- 
rent colours. 

The  direct  light  of  the  lamp  G,  as  well  as  that  re- 
flected by  the  mirror  and  passing  through  the  lens  <?,  is 


180 


THE  WONDERS  OF  OPTICS. 


so  concentrated  as  to  project  a  brilliant  beam  of  light 
through  the  painted  slide,  which  being  in  the  conjugate 
focus  of  the  large  plano-convex  lens  d,  the  pictures  on 
the  glass  are  refracted  in  a  magnified  form  on  the  white 
cloth  p  Q. 

The  magic  lantern,  therefore,  consists  of  a  box  to  hold 
the  lamp,  a  concave  mirror,  and  a  convex  lens  to  concen- 
trate the  light  on  the  slide,  and  a  second  convex  lens  to 
throw  the  image  on  the  screen. 


THE  PHANTASMAGORIA. 


183 


CHAPTER  II. 

THE  PHANTASMAGORIA. 

The  phantasmagoria  may  be  described  as  a  perfected 
magic  lantern,  and  bears  the  same  relation  to  its  proto- 
type that  a  shilling  telescope  bought  in  the  Lowther 
Arcade  does  to  one  of  Dollond's  or  Ross's  field  glasses. 
The  position  of  the  spectators,  too,  is  different,  being  on 
the  other  side  of  the  scene  which  receives  the  magnified 
pictures,  already  described  when  speaking  of  the  magic 
laitern. 

The  phantasmagoria  lantern  is  generally  mounted  on 
a  stand  provided  with  castors  so  that  it  may  be  moved 
about  at  will.  It  consists  of  a  box  as  represented  in 
fig.  52,  inclosing  a  lamp  with  a  metallic  reflector,  the 
bundle  of  rays  being  sent  through  the  centre  of  the 
tube  containing  the  slide  and  lenses,  as  before  described. 
The  chimney  serves  to  carry  off  the  products  of  com- 
bustion generated  by  the  lamp.  In  fig.  53  we  have 
shown  the  interior  of  the  tube  containing  the  lenses. 
Between  this  tube  and  the  body  of  the  lantern  there  is  a 
space  within  which  slide  the  glasses  whereon  are  painted 
the  figures  and  landscapes  that  are  to  be  thrown  on  the 
white  screen.  The  luminous  rays  given  off  by  the  re- 
flector in  the  interior  of  the  lantern  pass  through  a 
plano-convex  lens  placed  with  the  flat  side  outwards. 
In  front  comes  the  double  convex  lens,  or  object-glass, 
which  can  be  moved  backwards  and  forwards  by  means 


184 


THE  WONDERS  OF  OPTICS. 


of  a  rack  and  pinion.  There  is  also  a  movable  diaphragm, 
which  is  wo^k^.d  with  a  couple  of  cords,  by  palling  which 
the  apevture  is  made  larger  or  smaller  at  will.  By 
moving  ihe  lantern  backwards  and  forwards,  working 


Fig.  52.— The  Phantasmagoria. 


the  rack  and  pinion  and  the  diaphragm  at  the  same 
time,  the  view  seen  by  the  spectator  seems  to  advance 
and  recede.  The  pictures  are  painted  on  glass  with 
transparent  colours,  the  glasses  being  generally  about 
five  inches  in  diameter.  To  render  the  illusion  perfect 
it  is  necessary  that  the  spectator  should  be  placed  in  a 
partially  dark  room,  being  separated  from  the  operator 
by  the  screen  already  mentioned.  Everything  being 
ready,  the  spectators  having  but  little  notion  of  the 


THE  PHANTASMAGORIA. 


185 


distance  of  the  screen,  a  very  small  picture  is  shown  to 
them  first,  the  illumination  being  reduced  to  a  mini- 
mum by  pulling  the  cords  which  act  on  the  diaphragm. 
The  little  picture  first  seen  by  them  will  appear  to  be 
situated  at  an  enormous  distance ;  but  as  the  lantern  is 
brought  almost  imperceptibly  nearer  to  the  screen,  the 
image  appears  to  advance  towards  them  in  a  very  sur- 


Fig.  53.— The  Fantascope. 

prising  manner,  at  last  appearing  almost  as  if  it  were 
going  to  fall  upon  the  spectators. 

Robertson,  an  English  optician  who  was  settled  in 
Paris  some  fifty  years  since,  was  one  of  the  first  to  ex- 
hibit the  phantasmagoria  with  success.  In  order  to 
obtain  the  best  results  he  used  a  room  some  sixty  or 
eighty  feet  long,  and  twenty-four  wide,  which  he  hung 
entirely  with  black.  Of  this  a  strip  twenty-five  feet  long 


186 


THE  WONDERS  OF  OPTICS. 


was  cut  off  and  devoted  to  the  manipulation  of  the 
phantasmagoria.  This  portion  of  the  apartment  was 
separated  from  the  spectators  by  a  white  calico  screen, 
tightly  strained  from  side  to  side,  and  at  first  concealed 
from  view  by  a  black  curtain.  The  calico  screen, 
which  was  about  twenty  feet  square,  was  well  soaked  in 
a  mixture  of  starch  and  fine  gum  arabic,  in  order  to  ren- 
der it  semi-transparent.  The  floor  was  raised  about 
four  or  five  feet  at  one  end  in  order  that  the  whole  of  the 
spectators  might  have  a  free  and  uninterrupted  view  of 
what  was  going  on. 

It  is  undoubtedly  to  Robertson  that  we  owe  most  of 
the  improvements  in  the  phantasmagoria.  The  success 
of  his  performances  in  Paris  during  the  first  Revolution 
has  never  been  equalled  by  any  similar  exhibition.  The 
enthusiasm  excited  amongst  the  Parisian  public  at  the 
time  surpassed  that  awakened  even  by  Cagliostro  and 
Mesmer.  The  spirit  which  guided  Robertson  in  exhi- 
biting these  wonders  was  totally  opposed  to  that  which 
animated  the  two  charlatans  just  mentioned.  Robertson, 
unlike  them,  sought  to  spread  the  notion  that  there  was 
nothing  occult  or  supernatural  in  the  marvels  he  exhi- 
bited, but  that  they  resulted  simply  from  the  applica- 
tion of  a  few  simple  laws  of  optics.  We  shall  presently 
.give  an  account  of  one  these  famous  seances,  which 
were  powerful  enough  to  distract  the  attention  of  the 
people  of  that  day  from  the  stormy  events  that  were 
going  on  around  them;  but  we  will  first  allow  our 
author  to  tell  the  story  of  his  experiments  in  optics  in 
his  own  words. 

"From  my  very  earliest  infancy,"  he  says  in  his 
Memoirs,  "  my  lively  and  passionate  imagination  caused 
me  to  be  dominated  over  by  the  marvellous  in  a  very 
powerful  manner.  Anything  that  seemed  to  go  beyond 
nature  in  any  way,  excited  in  me  an  ardour  which  then 
appeared  to  me  capable  of  overcoming  all  obstacles  in 


THE  PHANTASMAGORIA. 


187 


order  to  realize  the  effects  I  had  conceived.  Father 
Kircher,  it  was  said,  believed  that  the  magic  lantern 
was  the  invention  of  the  Evil  One.  All  the  w^orse  for 
Father  Kircher,  who  was  gifted  with  a  great  intellect, 
and  many  persons  were  tempted  to  say  that  he  might 
possibly  have  some  cause  for  believing  in  the  diabolical 
origin  of  a  simple  optical  instrument.  But  as  the  writer 
who  has  thus  reproached  Father  Kircher  with  too  much 
credulity  has  not  cited  those  passages  of  the  work  in 
which  this  statement  maybe  found,  I  did  not  think  seri- 
ously of  the  matter.  "Who  has  not  in  his  younger  days 
believed  in  witches,  hobgoblins,  and  compacts  with  the 
devil?  I  know  I  did,  and  worse;  for  1  imagined  and 
fully  believed  that  an  innocent  old  woman  who  was  a 
neighbour  of  ours,  really  had  dealings  with  Lucifer,  as 
every  one  asserted.  I  even  went  so  far  as  to  envy  her 
the  power  of  conferring  with  the  Evil  One,  and  once 
shut  myself  up  in  my  room  with  an  unhappy  live  cock, 
whose  head  I  cut  off  in  the  most  barbarous  manner, 
having  heard  that  that  was  the  most  approved  manner 
of  summoning  into  one's  presence  the  great  head  of  all 
the  demons.  I  waited  for  him  several  hours,  calling 
on  him  to  appear,  threatening  to  deny  his  existence  for 
the  future  if  he  did  not  appear,  but  all  to  no  purpose. 
The  books  on  magic  and  the  black  art  that  I  had  read 
had  completely  turned  my  head.  I  believed  everything 
that  was  in  them,  and  I  desired  ardently  to  perform  the 
wonders  they  described,  even  with  the  aid  of  the  devil. 
The  Magia  Natiiralis  of  Porta,  and  the  Recreations  of 
Midorge,  which  treated  simply  of  natural  phenomena, 
had  no  effect  upon  me,  but  I  was  at  last  obliged  to  fall 
back  on  the  principles  involved  in  them,  in  order  to 
create  the  diabolical  appearances  I  had  sought  after  in 
what  I  considered  a  truly  supernatural  manner,  until  at 
last  my  dwelling  became  a  true  Pandemonium. 

"It  is  only  our  grandmothers,  it  has  been  said  for  a 


188 


THE  WONDERS  OF  OPTICS. 


long  time,  who  believe  in  magic,  witches,  and  super- 
natural appearances  ;  but  the  statement  is  hardly  true, 
seeing  how  easily  the  country  people  fall  a  prey  to  the 
first  cheat  who  chooses  to  invest  himself  with  super- 
natural powers.  We  have  sufficiently  ridiculed  the 
superstitions  of  the  ancients,  and  numberless  instances 
may  be  adduced  which  are  a  shame  to  their  intelligence, 
and  which  gives,  so  to  speak,  a  denial  to  the  stories  we 
have  heard  of  their  high  state  of  civilization.  But  I 
balieve,  if  we  were  to  make  a  collection  of  all  the  stories 
of  ghosts,  of  mysterious  appearances,  of  communications 
between  the  living  and  the  departed,  of  the  discoveries 
of  hidden  treasures,  &c,  &c,  which  have  taken  place 
even  since  the  Revolution,  before  whose  power  so  many 
dark  things  have  been  brought  to  light,  the  collection 
would  hardly  be  less  bulky  than  that  of  the  ancient  su- 
perstitions now  happily  passed  away." 

Robertson  then  goes  on  to  take  great  credit  to  him- 
self for  showing  the  world  that  all  the  superstitions  con- 
cerning ghosts,  spectral  appearances,  and  other  illusions 
of  a  similar  nature,  were  to  be  easily  accomplished,  by 
simply  studying  natural  laws.  He  appears  first  to  have 
begun  his  optical  experiments  with  the  solar  microscope, 
and  we  hear  of  his  landlord  taking  an  action  against  him 
to  recover  damages  for  having  pierced  the  doors  of  his 
rooms  with  innumerable  holes.  He  studied  the  subject 
both  theoretically  and  practically  for  many  years,  in 
company  with  his  friend  Villette,  and  at  last  announced 
a  public  seance  at  the  Pavilion  de  l'Echiquier  at  Paris. 
A  multitude  of  advertisements  and  prospectuses, 
written  in  the  high-flown  style  of  the  time,  were  issued, 
and  distributed  throughout  the  city.  The  newspapers 
of  the  day  are  full  of  accounts  of  the  extraordinary 
impression  made  on  the  minds  of  the  Parisians  by 
Robertson's  wonderful  exhibition.  The  old-fashioned 
word  magic  lantern  was  quite  abandoned,  and  the  new 


THE  PHANTASMAGORIA. 


189 


and  high  sounding  Greek  appellation,  "phantasmagoria," 
was  heard  issuing  from  every  one's  mouth.  There  is 
an  amusing  account  given  of  Robertson's  exhibition  in 
one  of  the  contemporary  journals,  written  by  Poultier, 
one  of  the  representatives  of  the  people.  He  says:  "A 
decemvir  of  the  republic  has  said  that  the  dead  return 
no  more,  but  go  to  Robertson's  exhibition  and  you  will 
soon  be  convinced  of  the  contrary,  for  you  will  see  the 
dead  returning  to  life  in  crowds.  Robertson  calls 
forth  phantoms,  and  commands  legions  of  spectres.  In 
a  well-lighted  apartment  in  the  Pavilion  de  l'Echiquier 
I  found  myself  seated  a  few  evenings  since,  with  some 
sixty  or  seventy  people.  At  seven  o'clock  a  pale  thin 
man  entered  the  room  where  we  were  sitting,  and 
having  extinguished  the  candles  he  said:  4  Citizens  and 
gentlemen,  I  am  not  one  of  those  adventurers  and  impu- 
dent swindlers  who  promise  more  than  they  can  per- 
form. I  have  assured  the  public  in  the  Journal  de 
Paris  that  I  can  bring  the  dead  to  life,  and  I  shall  do 
so.  Those  of  the  company  who  desire  to  see  the 
apparitions  of  those  who  were  dear  to  them,  but  who 
have  passed  away  from  this  life  by  sickness  or  other- 
wise, have  only  to  speak,  and  I  shall  obey  their  com- 
mands.' There  was  a  moment's  silence,  and  a  haggard 
looking  man,  with  dishevelled  hair  and  sorrowful  eyes, 
rose  in  the  midst  of  the  assemblage  and  exclaimed,  '  As 
I  have  been  unable  in  an  official  journal  to  re-establish 
the  worship  of  Marat,  I  should  at  least  be  glad  to  see 
his  shadow.'  Robertson  immediately  threw  upon  a 
brasier  containing  lighted  coals,  two  glasses  of  blood,  a 
bottle  of  vitriol,  a  few  drops  of  aquafortis,  and  two 
numbers  of  the  Journal  des  Hommes  Libres,  and  there 
instantly  appeared  in  the  midst  of  the  smoke  caused  by 
the  burning  of  these  substances,  a  hideous  livid  phan- 
tom armed  with  a  dagger  and  wearing  a  red  cap  of 
liberty.    The  man  at  whose  wish  the  phantom  had 


190 


THE  WONDERS  OF  OPTICS. 


been  evoked  seemed  to  recognise  Marat,  and  rushed 
forward  to  embrace  the  vision,  but  the  ghost  made  a 
frightful  grimace  and  disappeared.  A  young  man  next 
asked  to  see  the  phantom  of  a  young  lady  whom  he  had 
tenderly  loved,  and  whose  portrait  he  showed  to  the 
worker  of  all  these  marvels.  Robertson  threw  on  the 
brasier  a  few  sparrow's  feathers,  a  grain  or  two  of 
phosphorus,  and  a  dozen  butterflies.  A  beautiful 
woman,  with  her  bosom  uncovered  and  her  hair  floating 
about  her,  soon  appeared,  and  smiled  on  the  young  man 
with  the  most  tender  regard  and  sorrow.  A  grave- 
looking  individual  sitting  close  by  me  suddenly  exclaimed 
6  Heavens  !  it's  my  wife  come  to  life  again/  and  he 
rushed  from  the  room,  apparently  fearing  that  what  he 
saw  was  not  a  phantom. 

A  Swiss  asked  to  see  the  shade  of  William  Tell.  The 
phantom  of  the  great  archer  was  evoked  with  apparent- 
ly as  much  ease  as  the  others.  Delille,  who  was 
present,  called  for  Virgil,  whose  Georgics  he  had  lately 
translated.  The  poet  appeared,  having  in  his  hand  a 
laurel  crown,  which  he  held  out  to  his  French  commen- 
tator. Many  other  equally  extraordinary  apparitions 
were  shown  at  the  will  of  various  individuals  in  the 
audience,  and  towards  the  end  of  the  evening  Robert- 
son showed  his  judgment,  and  under  very  difficult  cir- 
cumstances. A  royalist  who  was  present  asked  for  the 
phantom  of  Louis  XVI.,  the  appearance  of  which  would 
no  doubt  have  raised  a  tumult  amongst  so  many  red- 
hot  Republicans,  had  not  Robertson  replied  that  before 
the  18th  Fructidor,  the  day  on  which  the  French  re- 
public declared  that  royalty  was  abolished  for  ever,  he 
had  had  a  receipt  for  bringing  dead  kings  to  life  again, 
but  that  same  day  he  lost  it,  and  feared  that  he  should 
never  recover  it  again.  The  answer  was  said  to  have 
been  whispered  to  Robertson  by  his  friend  Ponthieu, 
who  saw  the  difficulty  he  was  in.    It  was  supposed  that 


THE  PHANTASMAGORIA 


191 


the  demand  was  prompted  by  an  agent  of  the  police, 
who  for  some  cause  had  a  spite  against  Robertson.  In 
any  case  the  affair  made  such  a  noise  that  the  next  day 
the  exhibition  was  prohibited  by  those  in  authority, 
and  seals  were  placed  upon  the  optician's  boxes  and 
papers.  The  exhibition  was,  however,  afterwards  al- 
lowed to  be  continued,  and  was  so  successful  that  it  had 
to  be  transferred  to  the  old  Capuchin  convent  near  the 
Place  Vendome. 

The  whole  of  Paris  rang  with  eulogiums  upon  Robert- 
son's wonderful  exhibition  at  the  Capuchin  Convent. 
He  had  purposely  chosen  the  abandoned  chapel,  which 
was  in  the  middle  of  a  vast  cloister  crowded  with  tombs 
and  funereal  tablets.  It  was  approached  by  a  series  of 
dark  passages,  decorated  with  weird  and  mysterious 
paintings,  and  the  very  door  was  covered  with  hierogly- 
phics. The  chapel  itself  was  hung  with  black,  and  was 
feebly  illuminated  by  a  single  sepulchral  lamp.  The 
whole  assembly  involuntarily  remained  grave  and  silent, 
and  it  was  only  when  the  first  preparations  for  the  ex- 
hibition were  made,  that  the  audience  broke  into  a  low 
murmur.  Robertson  commenced  wTith  an  address  on 
sorcery,  magicians,  witches,  ghosts,  apd  phantoms,  and, 
having  worked  the  spectators  up  to  the  proper  pitch,  he 
suddenly  extinguished  the  single  antique  lamp  already 
mentioned,  plunging  the  assembly  into  perfect  darkness. 
Then  there  arose  a  storm  of  rain,  wind,  thunder,  and 
lightning.  The  bells  tolled  lugubriously  as  if  summon- 
ing the  dead  from  their  tombs  beneath  the  feet  of  those 
present ;  the  wind  whistled  mournfully,  the  rain  fell  in 
torrents,  the  thunder  rolled,  and  the  lightnings  flashed. 
But  suddenly  above  all  this  confusion  were  heard  the 
sweet  notes  of  a  harmonium,  and  in  the  far-off  distance 
the  sky  was  seen  clearing  gradually.  A  luminous  point 
then  made  its  appearance  in  the  midst  of  the  clouds, 
which  gradually  became  the  figure  of  a  man,  increasing 


192 


THE  WONDERS  OF  OPTICS. 


in  size  every  instant,  until  it  seemed  to  be  about  to  pre- 
cipitate itself  on  to  the  spectators.  A  man  in  the  front 
row  was  so  frightened,  that  he  uttered  a  scream  of  ter- 
ror, when  the  phantom  instantly  disappeared.  A  series 
of  spectres  then  issued  suddenly  from  a  cave.  The 
shades  of  great  men  crowded  together  round  a  boat 
floating  on  a  black  and  sluggish  river,  which  the  spec- 
tators had  no  difficulty  in  identifying  as  the  Styx.  The 
shadows  gradually  disappeared  in  the  distance,  getting 
smaller  and  smaller  until  they  became  invisible. 

Robertson  was  extremely  careful  in  all  his  entertain- 
ments to  flatter  the  popular  ideas  of  the  day.  For  in- 
stance, one  of  his  most  famous  exhibitions  consisted  in 
a  picture  of  a  tomb,  in  the  middle  of  which  Robespierre 
issued.  The  figure,  as  usual,  walked  towards  the  spec- 
tators ;  but  when  apparently  within  a  few  yards  of  them, 
it  was  struck  down  by  lightning.  Voltaire,  Lavoisier, 
Rousseau,  and  other  popular  favourites  then  appeared 
on  the  scene,  and  disappeared  again  in  the  usual  man- 
ner. Robertson  generally  ended  his  entertainment  with 
an  address  something  like  the  following : — 

"  We  have  now  seen  together  the  wonderful  mysteries 
of  the  phantasmagoria.  I  have  unveiled  to  you  the 
secrets  of  the  priests  of  Memphis.  I  have  shown  you 
every  mystery  of  optical  science ;  you  have  witnessed 
scenes  that  in  the  ages  of  credulity  would  have  been  con- 
sidered supernatural.  You  have,  perhaps,  many  of 
you,  laughed  at  what  I  have  shown  you,  and  the  gentler 
portion  of  my  audience  have  possibly  been  terrified  at 
many  of  my  phantoms;  but  I  can  assure  you,  whoever 
you  may  be,  powerful  or  weak,  strong  or  feeble,  believers 
or  atheists,  that  there  is  but  one  truly  terrible  spec- 
tacle— the  fate  which  is  reserved  for  us  all;"  and  at 
that  instant  a  grisly  skeleton  was.  seen  standing  in  the 
middle  of  the  hall  (fig.  54). 

Even  in  those  unbelieving  days,  when  scepticism  of 


THE  PHANTASMAGORIA. 


195 


every  sort  was  riding  rough-shod  over  the  French  people, 
Robertson  had  the  greatest  difficulty  in  disclaiming  all 
approach  to  the  possession  of  supernatural  powers. 
Day  after  day  he  received  applications  from  all  quarters 
to  reveal  the  secrets  of  the  past,  present,  and  future,  to 
describe  events  that  were  passing  in  other  countries; 
and  it  frequently  happened,  that  after  his  entertain- 
ments, he  would  be  asked  by  several  members  of  his 
auditory  to  assist  them  in  recovering  property  that  had 
been  lost  or  stolen  from  them.  In  the  latter  kind  of 
cases  he  generally  used  to  adopt  the  excellent  plan  of 
sending  his  would-be  clients  to  the  nearest  police-office. 


196 


THE  WONDERS  OF  OPTICS. 


CHAPTER  TIL 

OTHER  OPTICAL  ILLUSIONS. 

By  varying  the  disposition  of  mirrors,  prisms,  lenses, 
and  light,  an  infinite  number  of  the  most  surprising 
effects  may  be  shown,  with  a  comparatively  small 
amount  of  trouble  and  expense.  We  shall,  therefore, 
devote  this  chapter  to  the  explanation  of  a  large  number 
of  allusions,  .which  have  been  devised  by  Robertson  and 
other  adepts  in  the  art  of  honest  deception. 

One  of  Robertson's  most  famous  delusions  was  the 
"  Dance  of  Demons,"  an  effect  he  discovered  quite  acci- 
dentally. One  evening,  while  experimenting  with  the 
phantasmagoria,  he  suddenly  found  himself  in  the  dark, 
when  two  persons,  each  bearing  a  light,  crossed  the  room 
on  the  other  side  of  the  screen.  A  little  window  which 
happened  to  be  between  the  lights  and  the  screen,  imme- 
diately threw  its  double  image  on  the  cloth,  and  the 
method  of  multiplying  shadows  was  discovered. 

The  figures  used  in  this  experiment  are  cut  out  of 
fine  cardboard,  and  may  be  made  a  foot  high  or  there- 
abouts. They  are  placed  on  a  second  screen  in  front  of 
the  principal  one,  and  by  multiplying  the  lights,  as 
shown  in  fig.  55,  you  may  have  as  many  shadows  as 
you  please.  The  effect  is  much  heightened  if  the  figures 
are  cut  out  so  as  to  show  as  lights  when  thrown  on  the 
screen.  A  little  ingenuity  shown  in  the  arrangement  of 
the  distance  and  movements  of  the  lights,  will  produce  an 


OTHER  OPTICAL  ILLUSIONS. 


199 


endless  amount  of  amusing  effects.  Thus,  a  small  image 
of  the  principal  figure  may  be  produced  by  carrying  the 
second  light  to  a  great  distance,  and  the  lesser  figure 
may  be  easily  made  to  jump  over  the  former,  by  moving 
the  candle  in  a  semicircle  over  the  light  that  is  sta- 
tionary. It  is  only  necessary  to  recollect  that  whatever 
movements  are  made  by  the  lights,  the  shadows  of  the 
figures  follow  their  example.  With  a  little  ingenuity 
the  heads  and  limbs  of  the  figures  may  be  made  move- 
able ;  and  if  one  assistant  attends  entirely  to  the  working 
of  the  figures,  and  the  rest  to  the  lights,  an  infinite 
number  of  changes  may  be  carried  out.  If  mounted  in 
a  frame,  they  may  be  made  to  throw  somersaults,  fall 
down,  or  jump  up  in  the  air  at  will. 

A  knowledge  of  optics  will  often  serve  to  explain 
with  great  ease  the  tricks  played  by  conjurers  and  im- 
postors on  princes  and  other  great  people,  for  their  own 
vile  ends.  It  is  well  known  that  Nostradamus,  on  being 
consulted  by  Marie  de  Medicis  on  the  future  destiny  of 
France,  was  shown  by  him  in  a  mirror  events  that  left 
no  doubt  on  her  mind  that  she  would  one  day  share  the 
throne  of  the  Bourbons.  These  illusions  were  possibly 
effected  in  the  following  manner,  and  may  be  readily 
understood  by  reference  to  fig.  56. 

The  throne  in  the  first  chamber  is  reflected  in  a  mirror 
concealed  in  the  canopy  overshadowing  a  second  mirror, 
placed  carelessly  on  a  table  in  the  room  in  which  the 
Princess  and  astrologer  are  standing.  The  arrangement 
of  the  mirrors  is  such  that,  on  looking  into  the  smaller 
glass,  the  Princess  sees  all  that  is  going  on  in  the  adjoin- 
ing chamber.  The  very  fact  of  her  consulting  Nostra- 
damus on  her  future  fate,  shows  that  under  certain  cir- 
cumstances, at  least,  this  clever  woman  was  as  silly  as 
a  child.  It  is  not,  therefore,  to  be  supposed  that  she 
would  notice  that  the  mirror  she  was  looking  into  was 
inclined  at  such  an  angle  that  it  could  not  reflect  her 


200 


THE  WONDERS  OF  OPTICS. 


beautiful  face.  Nothing  could  be  more  natural,  either, 
than  that  this  magic  looking-glass  should  be  placed  on  a 
dai's,  and  shaded  by  a  canopy.  Nostradamus,  who  was 
a  shrewd  man,  could  no  doubt  pretty  well  see  the  course 
that  events  would  take,  and  must  consequently  have 
felt  quite  safe  in  showing-  the  Princess  the  throne  of 
France  occupied  by  Henry  of  Navarre.  This  was  not 
the  first  time  that  the  rulers  of  the  earth  were  duped  by 
so-called  magicians,  who  possessed  the  knowledge  that 
th?  angle  of  reflection  was  always  equal  to  the  angle  of 
refraction. 

Wd  may  also  mention,  while  speaking  on  this  subject, 
the  adventure  of  the  Emperor  Alexander  of  Russia,  cl 
propos  of  a  singular  optical  experiment  at  which  he  was 
present,  which  had  for  its  end  the  changing  of  a  man 
into  a  wil  1  animal,  or  vice  versa.  Certain  cynics  will 
possibly  say  that  this  is  by  no  means  difficult,  and  that 
it  is  an  event  that  happens  every  day  ;  but  the  clever 
trick  at  which  Alexander  was  so  astonished  was  not 
moral  but  purely  physical.  After  having  gained  much 
money  and  fame  in  France,  Robertson  directed  his  steps 
towards  Him  burg,  where  the  Emperor  was  at  that  time 
stopping.  H3  performed  before  the  Czar  an  experi- 
ment that  puzzled  his  Majesty  beyond  endurance.  He 
showed  him  a  man  upon  whose  shoulders  he  saw  succes- 
sively the  head  of  a  calf,  a  lion,  a  tiger,  a  bear,  and  a 
whole  menagerie  of  other  animals.  At  last,  the  Czar 
could  stand  it  no  longer,  and  he  suddenly  rose,  put  his 
shoulder  against  the  partition,  and  brought  the  whole 
to  the  ground  with  a  loud  crash,  just  at  the  moment 
that  the  confederate  was  assuming  the  form  of  a  goat. 
If  our  readers  would  like  to  join  the  Czar  in  his  dis- 
covery of  the  manner  in  which  the  trick  was  performed, 
they  can  easily  do  so. 

The  room  in  which  this  trick  is  to  be  performed  should 
have  a  smaller  one  adjoining  it,  about  eight  feet  square. 


OTHER  OPTICAL  ILLUSIONS. 


203 


The  magician  in  the  first  place  shows  the  small  apart- 
ment to  the  spectator,  who  perceives  that  it  contains 
nothing  but  an  empty  chair  placed  against  the  wall. 
The  partition  between  the  two  rooms  is  provided  with 
a  small  hole,  covered  with  glass,  exactly  opposite  the 
chair,  and  at  about  the  ordinary  height  of  the  eyes. 
On  the  inner  side  there  are  two  grooves,  in  which  slide  a 
block  of  wood  containing  a  prism,  as  shown  in  fig.  57, 


Fig.  57. — The  Arrangement  of  the  Reversing  Prism. 

which  may  be  quickly  and  easily  replaced  by  a  piece  of 
plane  glass.  On  looking  through  this  opening,  the 
spectator  sees  a  man  sitting  in  a  chair,  but  suddenly, 
without  any  apparent  cause,  the  man  changes  into  a  goat, 
a  sheep  or  some  other  animal.  The  sudden  replacing 
of  the  prism,  which  takes  place  without  the  spectator 
perceiving  it,  causes  him  to  see,  not  the  floor  with  the 
man  and  chair  upon  it,  but  the  ceiling,  which  is  carpeted 
exactly  in  the  same  way,  and  is  provided  with  a  pre- 
cisely similar  chair,  upon  which  is  placed  a  goat  or  any 
other  animal. 

While  looking  at  the  goat,  the  plane  glass  is  substi- 
tuted for  the  prism,  and  the  man  reappears ;  another 


204 


THE  WONDERS  OF  OPTICS. 


movement  of  the  prism,  and  he  changes  into  a  sheep, 
a  figure  of  a  sheep  having  in  the  meantime  replaced  that 
of  the  goat.  Of  course  it  is  necessary  not  merely  to  have 
the  walls,  floors,  and  chairs  precisely  alike,  but  they 
must  each  occupy  the  same  relation  to  each  other.  If 
it  is  desirable  only  to  change  the  head,  it  is  simply 
necessary  to  have  a  lay  figure  with  a  moveable  head, 
dressed  precisely  in  the  same  manner  as  the  living 
operator,  in  the  upper  portion  of  the  chamber.  At  the 
end,  by  the  substitution  of  the  empty  chair,  the  individ- 
ual may  be  made  to  disappear  entirely. 

There  may  often  be  seen  in  the  streets  of  London,  a 
man  showing  a  wonderful  instrument,  consisting  of  a 
telescope  cut  in  two,  the  two  portions  being  separated 
from  each  other  by  an  interval  of  three  or  four  inches. 
On  looking  through  the  instrument,  the  spectator  of 
course  sees  the  object  at  which  it  is  pointed ;  but  what 
is  his  astonishment  to  find,  that  when  the  showman 
places  a  brick  between  the  two  halves  of  the  instrument 
he  sees  just  as  well  as  before.  The  showman  generally 
informs  him  that  the  instrument  in  question  has  such 
powerful  lenses,  that  it  will  not  only  see  through  a 
brick,  but  even  through  a  policeman's  head  if  it  hap- 
pened to  be  in  the  way ;  and  the  spectator,  having  paid 
his  penny,  goes  away  perfectly  mystified,  until,  like  the 
young  lady  who  believed  that  all  machinery  wTas  worked 
"by  a  screw,  somehow,''  he  comforts  himself  with  the 
idea  that  the  trick  is  performed  "by  a  mirror,  some- 
how." The  following  figure  will,  however,  soon  clear 
up  the  mystery. 

Let  F  m,  L  a  be  an  ordinary  telescope  tube,  to  be 
separated  in  the  middle  by  an  interval  large  enough  to 
insert  a  brick,  the  hand,  or  some  other  opaque  object. 
The  whole  is  fixed  on  a  stand,  consisting  of  a  square 
tube  with  a  couple  of  elbows  to  it.  Between  G  and  L 
a  mirror  (a)  is  placed  diagonally,  which  receives  the 


OTHER  OPTICAL  ILLUSIONS. 


207 


image  of  the  objects  to  be  looked  at.  This  mirror  sends 
the  image  downwards  to  another  placed  diagonally  at 
c,  a  third  being  placed  at  D,  and  a  fourth  at  B.  The 


Fig.  59.— How  to  see  through  a  Brick. 


horizontal  ray,  meeting  the  mirror  at  A,  is  consequently 
bent  downwards  to  c,  then  travels  horizontally  to  D, 
when  it  is  reflected  upwards  to  B,  in  which  it  is  seen  by 
the  eye.  Of  course  a  simple  tube  without  any  lenses 
at  all  would  serve  the  same  purpose,  but  the  fact  of  its 
being  a  telescope  serves  to  distract  the  attention  of  the 
too  curious  observer. 

Another  illusion  of  the  same  kind  is  often  practised 
at  fancy  fairs  and  bazaars,  when  a  spectator  looking 
into  what  he  supposes  to  be  an  ordinary  looking-glass, 
sees  his  companions  instead  of  himself.  The  way  in 
which  this  is  effected  is  very  simple.  A  looking-glass 
is  placed  diagonally  across  a  square  box,  the  apertures 
in  the  sides  being  so  arranged  that  the  spectator  does 
not  perceive  that  he  is  looking  into  a  glass  that  is  placed 
at  an  angle.    Of  course  the  exhibitor  endeavours  to 


208 


THE  WONDERS  OP  OPTICS. 


show  the  illusion  to  two  persons  at  once;  and  if  they 
are  strangers  to  each  other,  and  of  the  opposite  sex,  a 
great  deal  of  fun  is  made  out  of  the  trick.  A  showman 
at  Greenwich  made  an  immense  harvest  by  showing  two 
such  mirrors,  one  to  all  the  young  girls  who  wished  to 
see  their  future  husbands,  and  the  other  to  all  the  young 
men  who  wished  to  see  their  future  wives.  Of  course 
he  had  a  tolerably  good-looking  male  and  female  con- 
federate to  help  him.  With  a  couple  of  mirrors  placed 
back  to  back  in  a  square  case,  with  an  opening  on 
each  side,  the  illusion  is  still  more  perfect,  as  on  look- 
ing through  any  of  the  holes  the  box  seems  to  be  quite 
empty. 

The  "Speaking  Head"  trick  is  performed  on  this 
principle.  When  the  curtain  is  drawn  up,  the  audience 
perceive  an  apparently  living  head  placed  on  a  small 
three-legged  table,  the  curtain  at  the  back  of  the  stage 
being  quite  visible  through  the  legs.  By  and  by  the 
bodiless  head,  which  is  generally  painted  in  a  very  fan- 
tastic manner,  begins  to  speak,  answers  questions,  and 
ends  by  singing  a  song.  The  trick  is  performed  in  the 
following  way :  The  spaces  between  the  legs  are  filled 
with  a  looking-glass ;  consequently,  the  spectators  see 
the  reflection  of  the  curtains  at  the  sides  of  the  stage, 
which  are  made  exactly  like  those  at  the  back,  thus 
giving  the  table  the  appearance  of  standing  on  three 
slim  legs,  with  nothing  between.  Behind  the  looking> 
glass  there  is  of  course  plenty  of  space  for  the  body  of 
the  man  belonging  to  the  magical  head.  The  exhibitor 
naturally  takes  especial  care  never  to  pass  in  front  of 
the  table,  otherwise  the  lower  part  of  his  body  would  be 
reflected  in  mirrors. 

The  polemoscope  (from  two  Greek  words  signifying 
"  war"  and  "to  see")  is  another  instance  of  double 
reflection.    It  was  said  to  have  been  invented  by  Hel- 


810 


OTHER  OPTICAL  ILLUSIONS. 


211 


vetius,  about  1637.  Fig.  60  will  .show  the  principle  of 
this  instrument. 

The  luminous  rays  coming  from  a  distant  object  are 
received  upon  an  inclined  mirror,  which  is  elevated 
above  the  parapet  of  a  fortification,  and  are  reflected 
downwards  to  a  second,  which  is  placed  at  a  correspond- 
ing angle.    If  necessary,  lenses  can  be  interposed,  so  as 
to  give  a  magnified  view  of  the  distant  object  that  is 
being  examined.    By  means  of  such  an  instrument,  the 
movements  of  the  enemy  can  be  followed  without  danger, 
the  apparatus  being  generally  of  small  size,  and  not 
attracting  notice.    Amongst  the  varieties  of  this  instru- 
ment, is  one  whose  use  is  readily  seen  by  inspecting  fig. 
61,  by  which  it  seems  to  be  perfectly  possible  to  see 
with  safety  all  that  is  going  on  outside  the  door  of  the 
house  without  being  perceived.    The  line  of  the  mirrors 
in  this  case  is  at  right  angles  to  that  of  the  polemoscope 
in  fig.  60.    Amongst  the  different  varieties  of  polemo- 
scope which  have  been  invented,  may  be  mentioned  a  re- 
flecting opera- glass,  which  was  greatly  used  by  the 
beaux  and  dandies  of  the  last  century.    In  the  tube  of 
this  instrument  was  inserted  an  inclined  mirror,  which 
allowed  the  spectator  to  point  his  glass  in  quite  a  dif- 
ferent direction  to  that  of  the  object  he  was  really 
looking  at.    In  fact,  it  was  constructed  somewhat  on 
the  same  principle  as  the  Herschellian  or  Newtonian 
telescope,  and  enabled  the  possessor,  while  apparently 
enjoying  the  play,  to  observe  all  that  was  going  on  in 
the  boxes  or  pit  of  the  theatre.    Years  ago,  there  was 
a  little  instrument  of  a  similar  kind,  sold  for  a  penny 
in  the  streets  of  London,  which  consisted  of  a  morsel  of 
looking-glass  set  at  an  angle,  in  a  pill-box,  and  which 
gave  the  possessor  the  power  of  seeing  all  that  was 
going  on  behind  him.    Persons  who  wear  dark  pre- 
servers are  often  in  the  habit  of  observing  all  that  is 


212 


THE  WONDERS  OP  OPTICS. 


going  on  behind  their  backs  by  the  reflection  seen  in 
the  corner  of  their  glasses. 

Such  are  the  principal  optical  recreations  founded  on 
the  reflecting  and  refracting  properties  of  mirrors  and 
lenses.  We  shall  end  this  chapter  by  appending  to  it 
the  description  of  a  few  additional  optical  amusements 
that  are  quite  within  the  reach  of  the  amateur. 

If  the  reader  is  in  possession  of  a  concave  mirror,  it 
may  be  made  the  means  of  performing  a  number  of 
amusing  experiments.  In  front  of  it  is  placed  a  plaster 
head,  a  skull  or  any  other  object,  mounted  on  wheels 
and  running  along  a  grooved  platform,  which  is  natu- 
rally kept  perfectly  concealed  from  the  spectators.  The 
mirror  is  slightly  inclined,  so  as  to  reflect  the  image  of 
the  object  at  an  angle  to  the  observer's  eye.  By  run- 
ning the  cast  backwards  and  forwards,  it  will  have  the 
appearance  of  advancing  and  retiring  from  the  specta- 
tor in  a  very  imposing  manner.  A  dagger  may  be  sub- 
stituted for  the  cast,  and  by  being  made  to  work  up  and 
down  on  a  pivot,  will  have  the  appearance  of  striking 
at  the  spectator.  We  have  already  seen  that  an  ex- 
periment of  this  sort  had  such  an  effect  on  Louis  XIV. 
that  he  drew  his  sword  to  defend  himself  from  his 
imaginary  aggressor.  There  is  another  way  of  per- 
forming this  trick,  by  suddenly  illuminating  the  skull 
or  dagger  by  means  of  a  dark-coloured  box  containing 
a  light,  which  may  be  made  to  throw  its  reflections  on 
the  object,  by  sliding  it  along  a  couple  of  wires.  In 
the  case  of  the  dagger,  however,  the  hinged  arrange- 
ment will  be  found  more  effective. 

One  of  Robertson's  tricks  was  called  the  "Magic 
Box,"  and  he  astonished  a  numerous  party  of  visitors 
who  were  staying  at  a  country  house  to  which  he  had 
been  invited.  One  of  the  gentlemen  who  was  always 
boasting  of  his  freedom  from  superstitious  feelings  of 
any  kind,  had  had  several  arguments  with  Robertson  on 


Fkj,  6i.  —  protection  against  ill-natured  people.  213 


OTHER  OPTICAL  ILLUSIONS. 


215 


the  subject  of  apparitions,  and  the  latter  thought  that 
he  would  at  any  rate  surprise  his  strong-minded  friend 
by  an  easy  trick  or  two.  He  consequently  chose  as  his 
confederate  a  lady  to  whom  the  gentleman  had  been 
paying  great  attention  during  the  time  of  his  visit. 
Robertson  one  evening  mysteriously  delivered  a  small 
box  to  him,  which  he  was  to  place  upon  his  toilet  table, 
and  unlock  exactly  at  midnight.  The  gentleman  did 
so,  and  what  was  his  astonishment  to  see  the  face  of  the 
lady  with  whose  charms  he  had  been  so  deeply  impressed 
suddenly  spring  out  of  the  box.  His  look  of  terror  and 
surprise  was  evidently  too  much  for  Robertson's  confe- 
derate, who  burst  into  a  merry  peal  of  laughter,  leaving 
her  admirer  in  a  very  disconcerted  state. 

After  all  we  have  said  on  the  subject  of  mirrors,  it  is 
not  difficult  to  guess  how  this  trick  was  performed.  The 
box  in  question  was  painted  black  on  the  inside,  and 
contained  a  concave  mirror  placed  at  an  angle  of  45°. 
The  reflection  of  the  lady,  who  was  of  course  in  the  next 
room,  was  carried  by  means  of  several  plane  mirrors 
placed  in  boxes  communicating  with  each  other  through 
the  partition  of  the  room,  the  head  of  the  lady  only  be- 
ing strongly  illuminated,  the  rest  of  her  figure  not  ap- 
pearing by  being  kept  quite  dark. 

The  figures  reflected  from  smoke  are  extremely  sur- 
prising. To  perform  such  experiments  a  phantasmago- 
ria is  necessary.  The  focus  is  so  adjusted  that  the  dis- 
tant image  falls  just  above  a  brasier  containing  lighted 
charcoal.  Everything  being  ready,  a  few  grains  of  oli- 
banum  or  other  gum  are  thrown  on  the  coals,  and  the 
smoke  that  rises  immediately  affords  a  screen  for  the 
reflection  of  the  images  proceeding  from  the  phantasma- 
goria. If  the  amateur  is  not  the  possessor  of  a  magic 
lantern,  a  properly  arranged  concave  mirror  will  answer 
almost  the  same  purpose. 


216 


THE  WONDERS  OF  OPTICS. 


CHAPTER  IV. 

THE  PROPERTIES  OF  MIRRORS. 

Almost  every  one  in  his  younger  days  has  possessed 
and  broken  that  pretty  instrument  known  as  the  ka- 
leidoscope. His  researches  into  its  construction  no  doubt 
taught  him  that  it  consisted  of  a  cylindrical  tube  in  tin 
or  cardboard,  with  a  moveable  cap  at  one  end  and  a 
small  hole  at  the  other.  In  the  interior  of  the  tube 
were  found  three  long  glasses,  blackened  on  the  back, 
placed  at  an  angle,  and  kept  in  position  by  pieces  of 
cork.  The  moveable  cap  was  provided  with  two  cir- 
cular pieces  of  glass,  one  ground  and  the  other  trans- 
parent, between  which  were  placed  a  number  of  pieces 
of  coloured  glass.  On  holding  the  instrument  up  to 
the  light  and  looking  through  the  eye-hole,  a  beauti- 
fully coloured  star  was  seen  whose  form  and  hue 
changed  by  simply  shaking  the  tube. 

The  kaleidoscope  was  invented  by  Sir  David  Brewster, 
and  is  exceedingly  simple  in  principle.  We  all  know 
that  if  a  luminous  object,  such  as  a  taper,  is  placed  be- 
fore a  mirror,  it  gives  forth  rays  of  light  in  all  direc- 
tions. Amongst  these  luminous  rays,  those  that  fall  on 
the  surface  of  the  mirror  are,  of  course,  reflected  in  such 
a  manner  that  the  angle  of  reflection  is  equal  to  the 
angle  of  incidence.  Ir  another  mirror  be  placed  at  right 
angles  to  the  first,  and  an  object  be  put  in  the  angle, 
the  image  of  it  will  be  multiplied  four  times.  If  the 
angle  be  diminished  to  60°,  six  reflections  will  be  seen, 


THE  PROPERTIES  OF  MIRRORS. 


217 


and  so  on.  A  symmetrical  figure  is  constantly  obtained, 
forming  in  one  case  a  cross  composed  of  four  similar 
portions  ;  in  the  other  a  triple  star,  the  halves  of  each 
ray  being  similar.  It  is  the  symmetry  of  the  figure  that 
gives  the  pleasing  effect.  In  the  ordinary  kaleidoscope 
the  angle  made  by  the  reflecting  surfaces  is  thirty  de- 
grees, and  a  star  of  six  rays  is  formed,  the  halves  of 
each  ray  being  alike.  The  figures  formed  in  the  ka- 
leidoscope are  simply  endless  ;  and  if  the  space  between 
the  glasses  in  the  moveable  cap  be  filled  with  bits  of 
opaque  as  well  as  transparent  substances,  the  varieties 
of  light  and  shade  may  be  added  to  those  of  colour.  It 
was  at  one  time  the  fashion  to  copy  the  images 
formed  in  the  kaleidoscope  as  paterns  for  room  papers, 
muslins,  curtains,  shawls,  and  other  similar  fabrics,  but 
thanks  to  the  spread  of  artistic  taste  in  this  country  the 
lecorative  designer  now  relies  more  on  his  own  talent 
than  any  aid  he  may  receive  from  optical  instruments. 

Plane  mirrors,  as  we  have  seen,  reflect  objects  up- 
right and  symmetrical,  reversing  only  the  sides.  Con- 
cave mirrors  reverse  them,  and  if  they  are  not  placed 
exactly  in  the  proper  focus,  distort  them  by  makingone 
portion  appear  smaller  than  the  other ;  while  convex 
mirrors  reflect  them  in  an  upright  position,  but  also 
similarly  slightly  distorted.  But  when  the  mirror  is 
not  a  portion  of  a  sphere,  like  those  whose  properties 
we  have  been  considering,  the  distortion  is  increased  to 
so  great  an  extent  as  to  deform  the  object  so  that  it  is 
difficult  to  recognise  its  nature  from  its  reflection.  We 
all  know  the  distortion  that  our  face  undergoes  when 
reflected  from  the  shining  surface  of  a  teapot  or  spoon, 
and  the  cylindrical  mirrors  that  hang  in  the  shop  win- 
dows of  many  opticians  are  the  source  of  much  amuse- 
ment to  the  passers  by,  whose  physiognomies  are  shown 
to  them  either  lengthened  to  many  times  their  natural 
size,  or  widened  to  an  extent  that  is  ludicrously  hide- 


218 


THE  WONDERS  OF  OPTICS. 


ous,  according  to  the  position  in  which  the  mirror  is 
hung.  Such  distortions  are  known  to  opticians  as  ana- 
morphoses, from  two  Greek  words  signifying  the  de- 
struction of  form  ;  and  distorted  drawings  used  to  be 
sold  at  one  time  which  when  reflected  from  the  surface 
of  the  cylindrical  mirror,  became  perfectly  symmetrical. 
Anamorphic  drawings  may  be  also  made,  which  when 
looked  at  in  the  ordinary  manner  appear  distorted,  but 
when  viewed  from  a  particular  point  have  their  symme- 
try restored  to  them.  With  a  little  knowledge  of  draw- 
ing, it  is  not  difficult  to  produce  these  in  great  variety. 

Suppose  the  portrait  in  fig.  62  to  be  divided  horizon- 
tally and  vertically  by  equidistant  lines  comprehended 
within  the  square  abcd, 


A  B 


.  C  D 
Fig.  62: 


Upon  a  second  piece  of  paper  draw  the  figure  shown 
in  fig.  63  in  the  following  manner.  Draw  the  horizon- 
tal line  a  b  equal  to  A  B  (fig.  62,)  and  divide  it  into  the 
same  number  of  parts.  Through  the  centre  draw  a 
perpendicular  line  to  V,  and  cross  it  by  a  line  e  d 
parallel  to  a  6.  Lastly,  draw  V  S  horizontal  to  e  d. 
The  length  of  the  two  lines  e  V  and  s  v  is  quite  arbitrary, 
but  the  longer  you  make  the  former  in  proportion  to  the 


THE  PROPERTIES  OF  MIRRORS. 


219 


latter  the  greater  will  be  the  distortion  of  the  drawing 
Now  draw  the  lines  v  1,  v  2,  v  3,  and  V  4,  and  join  g 
to  a.  Wherever  s  a  crosses  the  divisions  1,  2,  e  3,  4, 
and  6,  draw  a  horizontal  line,  parallel  of  course  with 
a  b.  You  will  thus  have  a  trapezium  abed  divided 
into  as  many  spaces  as  the  square  A  B  c  D  in  fig.  62, 
and  it  now  remains  to  fill  them  in  with  similar  portions 
of  the  figure.  Thus,  for  instance,  the  nose  is  in  the 
fourth  vertical  division,  starting  from  the  left,  and  in 
the  third  and  fourth  counting  from  the  top ;  in  order, 
therefore,  to  make  it  occupy  so  lengthened  a  space  it 
must  be  considerably  distorted  by  the  pencil.  It  will 
be  readily  seen  also  that  the  more  numerous  the  spaces 
into  which  the  square  is  divided,  the  easier  it  will  be 
to  draw  the  distorted  picture.  It  is  by  this  means  that 
the  anamorphosis  shown  in  fig.  63  has  been  drawn. 

The  next  thing  to  do  is  to  find  the  point  of  view 
from  which  we  can  see  the  figure  in  its  natural  propor- 
tions. This  will  be  found  to  be  at  a  distance  above  the 
point  v  equal  to  the  line  V  S.  In  order  to  complete  the 
experiment  it  is  simply  necessary  to  place  the  distorted 
picture  in  a  horizontal  position,  and  fix  a  piece  of  card- 
board vertically  at  the  point  v.  If  a  hole  be  punched 
in  it  at  a  distance  from  v  equal  to  S,  and  the  drawing 
be  looked  at  through  it,  the  whole  of  the  parts  will  fall 
into  symmetry  immediately. 

The  experiment  may  be  tried  first  with  fig.  63,  the 
hole  being  made  rather  large,  and  the  eye  placed  at  a 
distance  of  from  3  to  4  inches. 

It  Avould  be  difficult,  without  having  recourse  to 
geometrical  formulae,  to  explain  how  it  happens  that 
by  placing  the  eye  at  a  particular  point  the  distorted 
lines  of  the  drawing  become  symmetrical ;  but  perhaps 
a  mechanical  demonstration  will  help  to  make  this 
difficult  subject  a  little  plainer. 


220 


THE  WONDERS  OF  OPTICS. 


Draw  in  outline  any  figure  upon  a  piece  of  cardboard, 
and  make  a  series  of  pin-holes  along  the  most  prominent 
lines  of  the  drawing,  taking  care  that  they  are  pretty 


a    ±     2     &     s     s.  I 


Fig.  63. — Anamorphosis. 


close  together.  Place  the  perforated  card  in  a  vertical 
position  on  a  sheet  of  paper,  so  that  the  rays  from  a 
candle  or  lamp  may  fall,  on  the  flat  surface  beneath.  On 
looking  at  the  luminous  figure  formed  from  the  drawing, 
you  will  find  that  it  is  as  much  distorted  as  the  lady's 


THE  PROPERTIES  OF  MIRRORS. 


221 


head  in  fig.  63,  and  that  the  lower  you  place  the  candle 
the  greater  will  be  the  deformity.  You  may  if  you 
please,  trace  the  luminous  figure  on  the  paper,  and  the 
result  will  appear  distorted  when  looked  at  in  the  ordi- 
nary manner,  but  symmetrical  when  viewed  from  the 
point  at  which  the  flame  of  the  candle  was  placed. 

In  the  foregoing  experiments  we  have  spoken  of  the 
anamorphic  drawings  as  being  placed  in  a  horizontal 
position,  but  they  may  be  looked  at  just  as  well  verti- 
cally, the  card  with  the  hole  being  in  this  instance  hori- 
zontal. It  is  also  not  necessary  that  the  point  of  sight 
(v,  fig.  63)  should  be  in  the  centre  of  the  picture ;  it 
may  be  placed  at  one  side  or  the  other,  care  being 
taken  to  draw  all  the  divisional  lines  so  that  they  meet 
at  this  particular  spot.  A  few  experiments  with  a  can- 
dle and  a  perforated  figure  will  soon  show  the  student 
how  to  accomplish  this. 

Anamorphoses  by  reflection  may  be  prepared,  if  this 
principle  is  carried  out,  which  appear  a  mass  of  con- 
fused lines  until  they  are  reflected  in  a  cylindrical  mir- 
ror. Formerly  opticians  were  accustomed  to  construct 
anamorphoses  which  became  symmetrical  pictures  when 
viewed  in  a  conical  mirror ;  but  the  fashion  for  such 
toys  appears  to  have  gone  out.  Such  drawings  were 
extremely  difficult  to  make,  and  the  mirrors,  having  to 
be  ground  and  polished  with  great  care,  were  very  ex- 
pensive. 

Some  experimentalists  have  carried  the  subject  so 
far  that,  by  looking  at  the  drawing  of  an  object  in  par- 
ticular positions,  it  changed  into  quite  a  different  sub- 
ject. In  the  cloister  of  an  abbey  that  once  existed  in 
Paris,  there  were  two  anamorphoses  of  this  kind.  They 
were  the  work  of  a  certain  Father  Niceron,  who  hats 
left  behind  him  a  treatise  in  Latin  on  optical  wonders, 
entitled  Thaumaturgus  Opticus,  which  contains  a  long 
essay  on  anamorphoses.    One  of  these  pictures  repre- 


222 


THE  WONDERS  OF  OPTICS. 


eented  St.  John  the  Evangelist  writing  his  Gospel;  the 
other  Mary  Magdalene.  When  looked  at  in  the  ordi- 
nary manner,  they  appeared  to  be  landscapes;  but 
when  the  observer  placed  himself  in  a  particular  posi- 
tion, they  changed  into  the  figures  we  have  mentioned. 


CHINESE  SHADOWS. 


228 


CHAPTER  V. 

CHINESE  SHADOWS. 

While  upon  the  subject  of  optical  wonders,  we 
should  hardly  be  forgiven  if  we  did  not  give  a  descrip- 
tion of  the  amusement  known  as  Chinese  shadows,  or 
Fantocini.  In  the  winter  time  it  is  difficult  to  pass 
through  any  of  the  large  thoroughfares  of  London  after 
nightfall,  without  seeing  a  crowd  admiring  the  popular 
fantocini  farces  of  the  "Broken  Bridge,"  or  "  Billy 
Button;  "  and  although  these  dramatic  exhibitions  are 
not  always  free  from  vulgarity,  they  are  received  with 
vociferous  applause  by  at  least  the  younger  portion  of 
the  audience. 

The  apparatus  for  the  exhibition  of  the  fantocini  is 
generally  very  simple.  The  screen  on  which  they  are 
shown  is  generally  made  of  calico  rendered  semi-trans- 
parent with  copal  varnish,  and  the  figures  are  cut  out 
of  cardboard.  Frames  containing  landscapes  and  scenes 
of  different  kinds  are  also  provided,  which  are  cut  out 
in  the  same  material.  The  dramatis  personce  are  gen- 
erally made  with  moveable  limbs,  which  they  throw  about 
in  the  most  unanatomical  manner,  and  the  showman  is 
often  endowed  with  ventriloquial  talents  of  no  mean  or- 
der. This  amusement  is  to  be  found  in  all  parts  of  the 
world,  from  the  Strand  and  Tottenham  Court  Road 
London,  to  the  streets  of  Algiers  and  Java.  A  graphic 
writer  in  the  Magasin  Pittoresque  gives  a  pleasant  de- 


224 


THE  WONDERS  OF  OPTICS. 


scnption  of  the  fantocini,  as  exhibited  at  the  Arabs' 
theatre  in  the  Mohammedan  quarter  of  the  city  of  Al- 
giers. It  was  on  the  occasion  of  the  feast  of  the  Bai- 
ram,  which  immediately  follows  the  termination  of  the 
Ramadan,  or  Mohammedan  Lent.  The  theatre,  which 
was  the  only  one  frequented  by  the  Arab  population, 
consisted  simply  of  a  long  vaulted  hall,  without  seats, 
boxes,  or  galleries ;  but  the  audience,  who  had  already 
been  there  some  time,  did  not  seem  to  regard  the  omis- 
sion as  of  any  consequence,  but  had  seated  themselves 
on  the  ground  with  great  coolness,  chatting  in  whispers, 
and  waiting  patiently  until  the  director  should  consider 
the  place  full  enough  to  begin  the  performance.  Half 
an  hour  elapsed,  and  the  spectators  still  chatted  on 
quite  unconcernedly ;  an  hour,  and  yet  there  was  no 
hissing  or  stamping  of  feet  from  the  grave  and  patient 
spectators.  At  last  they  reached  the  maximum,  and  a 
boy  came  forward  and  blew  out  the  few  lamps  with 
which  the  theatre  was  lighted,  leaving  them  to  smoulder 
away  with  a  perfume  that  was  certainly  not  Oriental  in 
its  character.  First  came  the  legend  of  the  Seven 
Sleepers;  then  Scheherazade  relating  her  bewitching 
stories  to  the  Sultan.  These  were  followed  by  Aladdin 
and  the  Wonderful  Lamp,  a  story  that  is  as  popular  in 
Algiers  as  it  is  in  London  or  Paris ;  the  whole  culmina- 
ting in  a  kind  of  burlesque,  in  which  a  great  deal  of 
gross  fun  was  mixed  up  with  a  number  of  rebellious  al- 
lusions. The  devil,  for  instance,  who  is  of  course  one 
of  the  members  of  the  troupe,  is  portrayed  as  a  French 
soldier,  bearing  a  cross  on  his  breast  like  an  ancient 
Crusader.  After  him  came  Carhageuse,  who  is  the 
buffoon  of  the  Eastern  stage,  and  who  makes  violent 
but  unsuccessful  love  to  a  charming  young  Jewess. 
There  was  a  poor  barber  who  was  raised  to  the  dignity 
of  grand  vizier,  his  successor's  head  being  cut  off  by  the 
yataghan  of  the  Oriental  Jack  Ketch,  to  the  great  de- 


CHINESE  SHADOWS. 


227 


light  of  the  people.  Then  a  wretched  Jew  receives  the 
bastinado,  amidst  vociferous  applause,  which  increases 
still  higher  when  the  ears  of  an  unhappy  Giaour  are  cut 
off  and  thrown  to  the  dogs.  Throughout  the  piece,  it 
is  of  course  the  Mussulman  who  always  triumphs,  like 
the  French  guards  at  the  Cirque  Imperiale,  or  the  Brit- 
ish grenadiers  at  old  Astley's.  The  performance  con- 
cluded with  a  grand  naval  battle  between  the  Moorish 
and  Spanish  fleets.  The  drum  as  usual  served  for  can- 
non, there  was  a  great  deal  of  .smoke  and  confusion, 
and  the  Christian  fleet  gradually  sank  under  the  con- 
tinuous fire  of  the  Mussulmans  amidst  the  plaudits  and 
bravos  of  the  crowd. 

In  Java,  the  subjects  of  the  fantocini  are  generally 
taken  from  the  native  mythology.  The  screen  on  which 
the  shadows  are  exhibited  is  ten  or  twelve  feet  long, 
and  five  feet  high,  and  the  figures  are  cut  of  thick 
leather,  their  limbs  being  moved  by  thin  pieces  of  nearly 
transparent  horn. 

In  fig.  64  we  see  another  kind  of  Chinese  shadows, 
in  which  the  lights  of  the  figure  are  cut  out.  These 
pictures  are  perfectly  unrecognisable  as  being  even  the 
basest  imitation  of  any  known  form  ;  but  when  their 
shadows  are  thrown  on  the  wall,  the  cut-out  portions 
show  us  lights,  whilst  those  that  have  been  left  form 
the  shadows.  On  the  Boulevard  des  Capucines,  at 
Paris,  there  used  to  be  a  man  who  managed  to  pick  up 
a  good  living  by  selling  these  candle  shadows.  Of  course 
he  used  to  carry  on  his  trade  of  an  evening,  and  with 
a  strong  lamp  he  would  throw  the  shadows  of  his 
figures  on  the  white  walls  of  the  houses,  or  the  blind  of 
a  shop  window,  or  even  on  the  pavement.  With  a  little 
care  and  ingenuity  a  number  of  these  amusing  cards 
may  be  easily  designed.  In  showing  them,  care  must 
be  taken  to  choose  the  best  distances  between  the  light 
and  the  paper,  and  between  this  latter  and  the  wall. 


228 


THE  WONDERS  OF  OPTICS. 


If  the  card  be  placed  too  close  to  the  wall,  the  resulting 
shadows  will  be  too  dark,  and  the  outlines  too  sharp  ; 
if,  on  the  contrary,  the  light  is  placed  too  far  off,  the 
outlines  become  confused,  and  the  proper  effect  is 
lost. 

Shadows  have  been  applied  before  now  to  the  propa- 
gation of  seditious  ideas.  "  In  1817,"  says  an  esteemed 
French  author,  u  one  winter's  night  we  were  all  sitting 
round  the  table  listening  to  my  father,  who  was  reading 
aloud  an  interesting  book  of  the  period,  when  a  friend 
of  our  family,  who  had  been  formerly  an  officer  of  the 
Empire,  entered  the  room.  He  was  a  serious,  upright, 
soldierly  man,  and  wore  his  coat  buttoned  up  to  his 
chin.  He  had  hardly  replied  to  our  salutations,  when 
he  drew  a  chair  to  the  table,  and  made  a  sign  with  his 
hands  and  eyes  that  plainly  indicated  silence  and  dis- 
cretion. There  was  something  in  the  expression  of  his 
countenance  that  seemed  to  show  that  he  had  something 
mysterious  in  store  for  us,  and  we  fully  expected  to  hear 
some  extraordinary  news,  or  to  see  him  bring  out  a 
Bonapartist  pamphlet  of  more  than  usual  importance. 
Our  surprise  was  consequently  great  when  we  saw  him 
slowly  unscrew  the  top  of  his  cane,  which  was  turned 
out  of  boxwood,  and  presented  nothing  very  remarkable 
either  in  form  or  material.  He,  however,  took  up  a 
copybook  which  was  lying  on  the  table,  placed  it  at  a 
certain  distance  from  the  lamp,  and  then  laid  upon  it 
the  little  piece  of  turned  boxwood.  At  first  we  noticed 
nothing  at  all  extraordinary,  and  he  smiled  at  our  want 
of  intelligence,  until  at  last  my  youngest  brother  cried 
out  suddenly,  6  Look  !  there's  the  head  of  Napoleon  V 
and  truly  enough,  we  found,  on  looking  more  attentively 
at  the  shadows  of  the  turned  knob  of  the  cane,  that  their 
profile  was  that  of  the  great  exile,  most  correctly  and 
clearly  portrayed.  The  old  captain's  face  lighted  up  at 
the  sight,  and  the  tears  came  into  his  eyes.    '  We  shall 


CHINESE  SHADOWS.  229 

see  him  again/  he  murmured  in  a  low  voice,  and  he 
hummed  the  burden  of  a  Bonapartist  song  then  in 
vogue.    During  the  rest  of  the  evening  he  was  very 


Cane. 

Fig.  65  —  Seditous  Toys. 


lively,  and  proved  to  us  most  conclusively,  that  before 
six  months  the  Crrande  Armee  would  be  revenged  for 


230 


THE  WONDERS  OF  OPTICS. 


their  defeat  at  Waterloo.  Some  weeks  after,  there  was 
hardly  a  soldier  in  the  town  that  did  not  possess  a  stick 
or  a  tobacco-pipe  stopper,  turned  in  this  fashion,  but 
one  day  a  panic  seized  everybody,  and  the  canes  and 
pipe  stoppers  were  all  burnt.,, 

Fig.  65  represents  historic  heads  cut  in  this  way. 
During  the  Shakespeare  Tercentenary  excitement,  a 
London  turner  made  quite  a  little  fortune  by  making 
heads  of  the  great  poet  on  the  same  principle. 


P0LY0RAMA— DISSOLVING  VIEWS — DIORAMA.  231 


CHAPTER  VI. 

POLYORAMA-^-DISSOLVING  VIEWS — DIORAMA. 

The  description  of  the  polyorama  naturally  follows 
that  of  the  phantasmagoria,  being  a  practical  applica- 
tion of  precisely  the  same  principles.  In  the  case  of 
the  polyorama,  however,  two  or  even  more  lanterns  of 
the  best  construction,  are  used.  There  are  therefore 
two  sets  of  lenses  identical  in  every  particular,  placed 
side  by  side,  in  the  same  line,  the  foci  of  both  being 
adjusted  for  the  same  spot,  so  that  the  images  refracted 
from  each  may  superpose  each  other  without  difficulty. 
In  each  instrument  there  are  the  same  pictures,  but 
they  differ  in  certain  particulars,  as  we  shall  see  pres- 
ently. 

In  the  phantascopes  shown  in  figs.  52  and  54  there 
are  two  sets  of  lenses ;  the  first  carries  a  glass  bearing 
the  image  of  a  skeleton  in  a  winding  sheet,  while  on  the 
glass  belonging  to  the  second  a  naked  skeleton  is  por- 
trayed. If,  therefore,  at  a  given  instant  the  first  lan- 
tern is  shut  off,  the  spectators  see  the  winding  sheet 
torn,  as  it  were,  suddenly  from  the  spectre  before  them. 
The  first  lantern  being  turned  on  once  more,  the  skele- 
ton is  instantly  reclothed  in  its  hideous  garb. 

It  is  of  course  not  necessary  always  to  choose  such 
horrible  subjects  for  representation,  as  it  is  possible 
to  produce  changes  of  a  much  more  agreeable  nature. 
For  instance,  a  volcano  may  be  depicted  during  its 


232 


THE  WONDERS  OF  OPTICS. 


tranquillity,  with  the  sun  shining  on  its  verdant  sides, 
and  surmounted  with  a  gently  rising  wreath  of  smoke. 
Then  it  may  be  shown  at  night,  with  its  crater  vomiting 
flames  and  red-hot  stones,  while  streams  of  lava  are 
flowing  beneath.  By  proper  mechanism,  one  lantern 
may  be  gradually  shut  and  the  other  as  gradually 
opened,  producing  an  effect  that  appears  perfectly 
natural,  from  the  gentle  change  which  takes  place. 
Daylight,  twilight,  and  moonlight  effects  may  be  easily 
made  to  succeed  each  other  in  their  proper  order,  and 
the  most  opposite  scenes  may  be  made  to  change  each 
other  by  proper  appliances.  Those  who  have  seen  the 
dissolving  views  at  the  Polytechnic,  know  what  effects 
are  produced  by  this  very  simple  means.  A  virgin 
forest  changes  to  a  crowded  church,  which  in  turn  dis- 
solves into  a  scene  on  the  Alps. 

The  diorama,  properly  so  called,  invented  by  the  il- 
lustrious Daguerre,  differs  completely  in  principle  from 
the  apparatus  we  have  just  been  describing.  As  its 
etymology  indicates,  the  pictures  shown  are  seen 
through.  As  in  the  case  of  the  polyorama,  there  are 
two  different  effects  painted  upon  the  cloth,  which  are 
brought  out  by  a  double  system  of  illumination. 

Fig.  66  will  show  the  way  in  which  these  changes  are 
managed.  The  large  picture,  which  is  hanging  verti- 
cally, is  painted  both  in  front  and  behind.  The  front 
is  illuminated  by  reflection  from  a  semi-transparent 
screen  placed  over  it,  which  receives  the  light  of  the 
floor  above.  The  back  is  lighted  from  the  windows  be- 
hind, which  are  provided  with  blinds  to  regulate  the 
am>unt  of  light.  The  effects  produced  by  the  diorama 
were  truly  marvellous,  and  Daguerre  had  a  special  ta- 
lent for  this  kind  of  painting.  His  famous  Midnight 
Mass,  which  was  exhibited  at  the  Regent's  Park,  was 
one  of  the  most  renowned  of  his  works.  The  scene  first 
represented  a  dark,  empty  church,  feebly  lighted  by  a 


POLYORAMA — DISSOLVING  VIEWS — DIORAMA.  235 


small  altar  lamp,  but  gradually  the  lights  appeared  here 
and  there,  worshippers  congregated  in  front  of  the  al- 
tar, filling  the  nave  and  aisles.  In  Paris  the  same 
scene  was  exhibited,  representing  the  interior  of  the 
Church  of  St.  Germain  l'Auxerrois  with  such  perfect 
reality,  that  a  countryman  actually  threw  a  halfpenny 
against  the  painted  canvas,  to  see  whether  he  were 
really  in  a  church  or  not. 

The  next  scene  represented  the  destruction  of  the 
village  of  Goldau,  near  Lucerne,  by  a  landslip.  First 
there  appeared  a  smiling  fertile  valley,  its  sides  crowned 
with  verdure ;  a  storm  gradually  rose,  the  rain  fell,  the 
wind  blew,  the  lightnings  flashed,  and  the  thunder  rolled 
in  the  distance.  Darkness  at  last  closed  in,  and  when 
the  sun  once  more  rose  over  the  valley,  nothing  was  to 
be  seen  but  a  mass  of  fallen  rocks. 


THE  WONDERS  OF  OPTICS, 


CHAPTER  VII. 

THE  STEREOSCOPE. 

Having  devoted  so  much  space  in  the  preceding 
chapters  to  optical  amusements  of  a  purely  recreative 
character,  it  is  only  right  that  we  should  now  say  a  few 
words  on  certain  instruments  of  a  less  frivolous  character 
than  those  we  have  lately  been  considering,  and  which 
deserve  at  our  hands  the  most  serious  attention.  We 
shall,  therefore,  in  the  present  chapter,  speak  of  an  in- 
genious instrument  which  serves  to  show  in  relief  the 
images  of  objects  depicted  on  a  flat  surface.  We  have 
already  seen,  that  although  we  have  two  eyes,  provided 
with  lenses  and  screens  by  means  of  which  the  images 
of  things  around  us  are  formed,  we  only  perceive 
a  single  object ;  and  the  student  has  no  doubt  long 
since  wondered  why  nature  has  bestowed  two  eyes  upon 
us,  when  only  one  would  have  apparently  served  the 
same  purpose.  This  question  was  for  a  long  time  a 
complete  puzzle  to  philosophers,  and  it  was  not  until 
Professor  Wheatstone  made  his  experiments  on  binocular 
vision  in  1838,  that  the  matter  received  a  satisfactory 
explanation.  He  showed  that  each  eye  receives  a  dif- 
ferent impression  of  any  object  upon  the  retina,  and  that 
it  is  in  consequence  of  the  union  of  these  slightly  dis- 
similar images  that  the  sensation  of  relief  is  experienced.' 
A  one-eyed  man  or  a  Cyclops  would  only  partially 
perceive  relief  in  the  objects  presented  to  his  view,  in 


THE  STEREOSCOPE. 


237 


consequence  of  a  single  image  being  sent  to  his  brain. 
He  would,  no  doubt,  after  examining  the  things  he  saw 
with  his  hands,  know  they  were  solid,  and  generally  see 
them  so  ;  but  if  a  new  object  were  presented  to  his  view 
he  would  have  some  difficulty  in  knowing  whether  it 
had  a  flat  surface  or  not. 


FA 


Fig.  67. 


The  principle  of  binocular  vision  may  be  explained 
as  follows  :  If  a  playing  die,  such  as  is  represented  in 
fig.  67,  be  held  out  at  arm's  length  in  the  position  indi- 
cated in  the  figure,  and  looked  at  first  with  the  left  eye 
and  then  with  the  right,  we  shall  find  that  in  the  first 
case  we  see  a  little  of  the  three  dots  on  the  left-hand 
side,  and  in  the  second  we  lose  sight  of  the  three  dots 
and  see  a  little  of  the  single  one  on  the  right-hand  side. 
The  images  seen  by  each  eye  are,  therefore,  slightly  dis- 
similar, and  it  stands  to  reason  that,  if  by  any  means  we 
can  combine  two  slightly  dissimilar  flat  pictures  of  a 
solid  object,  we  shall  see  it  in  relief.  This  was  proved 
practically  by  Professor  Wheatstone,  who  constructed 
an  instrument  capable  of  effecting  the  desired  union, 
and  which  has  since  been  called  the  stereoscope,  from 
two  Greek  words  signifying 4  to  see  solid. '  The  instrument 
remained  for  a  long  time  fallow,  so  to  speak,  from  the 
difficulty  of  drawing  two  pictures  that  should  be  iden- 
tical in  size  and  details,  although  dissimilar  in  the 
arrangement  of  their  perspective.  It  was,  therefore,  not 
until  photography  enabled  us  to  do  this  with  the  greatest 


238  THE  WONDERS  OF  OPTICS. 

ease  and  exactitude  that  the  stereoscope  became  com- 
mon. The  instrument  first  devised  by  Professor  Wheat- 
stone,  was  what  is  termed  a  reflecting  stereoscope,  and 
was  expensive  to  make  and  cumbrous  to  use.    It  was 


Fig.  68.— Stereoscope. 


modified  by  Sir  David  Brewster,  by  the  substitution  of 
prisms  for  reflectors,  and  was  thus  made  cheaper  and 
more  portable.  The  refracting  form  of  stereoscope  is 
so  familiar  to  most  people,  that  it  really  needs  no 
description.  It  will  only  be  necessary  to  mention  that 
the  prisms  used  in  the  eye-pieces  are  made  by  cutting 
a  double  convex  lens  in  two,  and  reversing  the  halves. 
They  are  so  placed  that  the  centre  of  each  prism  is  just 
in  the  centre  of  each  eye ;  but  as  the  eyes  of  "different 
people  vary  in  distance,  an  arrangement  is  generally 
added  so  that  the  eye-pieces  may  slide  from  side  to  side. 
Being  cut  from  lenses,  the  prisms  have  a  magnifying 
power;  consequently  other  means  are  provided  for 
sliding  them  up  and  down  to  suit  the  length  of  focus 
in  different  eyes. 

In  fig.  69  we  can  follow  the  path  of  the  rays  pro- 
ceeding from  each  picture,  and  reach  the  eyes  appa- 
rently from  a  spot  exactly  between  the  two. 

In  the  reflecting  stereoscope  two  mirrors  are  joined 


THE  STEREOSCOPE. 


239 


together  at  right  angles  to  each  other,  the  two  pictures 
being  placed  at  each  side,  at  a  distance  corresponding 
to  their  size.    The  reflecting  instrument,  although  not 


Fig.  69.— The  Principle  of  the  Refracting  Stereoscope. 

so  portable,  is  in  some  sort  superior  to  the  other, 
inasmuch  as  pictures  of  any  size  can  be  seen  by  it, 
whilst  in  the  smaller  instrument  the  size  of  the  photo- 
graph is  limited  by  the  distance  at  which  the  eyes  are 
placed. 

It  should  be  mentioned,  that  no  optical  instrument  of 
any  kind  is  absolutely  necessary  to  obtain  a  stereoscopic 
effect  from  two  suitable  drawings  or  photographs,  as  it 
is  quite  possible  by  a  little  management  of  the  eyes  to 
cause  the  two  images  to  combine  with  each  other.  Re- 
ferring again  to  fig.  67,  it  will  be  perceived  that  the 
two  figures  of  the  dice  are  about  an  inch  and  a  half 
from  each  other.  Holding  the  book  at  about  ten  inches 
from  the  eye,  they  are  viewed  by  squinting  strongly 
until  the  right  eye  looks  at  the  left  die,  and  the  left  eye 
at  the  right.  This  may  be  also  done  by  converging  the 
eyes  on  a  point  beyond  the  centre  of  the  figure,  which 
maybe  easily  done  by  looking  at  a  point  midway  between 
the  two.  In  both  cases  the  images  at  first  appear  dou- 
bled, and  we  see  four  dice,  but  a  little  practice  will  soon 
enable  you  to  cause  the  two  inside  images  to  coalesce, 


240 


THE  WONDERS  OF  OPTICS. 


and  so  give  the  effect  of  relief.  It  is  true  that  even  then 
three  images  are  seen,  but  the  eye  soon  grows  accus- 
tomed to  neglect  them  altogether.  This  habit  is  a  very 
pleasant  acquirement  for  the  London  flaneur,  who  can 
thus  see  in  perfection  the  numberless  stereoscopic  views 
now  shown  in  our  shop-windows  without  the  intervention 
of  an  instrument  of  any  kind. 

The  method  of  photographing  subjects  for  the  stereo- 
scope is  very  simple,  and  consists  in  taking  two  views 
of  the  object  to  be  depicted,  from  two  different  points. 
According  to  the  distance  of  these  points  from  each 
other,  so  will  the  resulting  pictures  appear  in  greater  or 
less  relief.    This  is  readily  seen  in  some  stereoscopic 
portraits  which  have  been  taken  at  a  large  angle,  and 
consequently  show  such  increased  relief  as  to  produce 
distortion.    Theoretically,  the  interval  of  the  two  points 
of  view  ought  to  be  two  inches  and  a  half,  that  being 
the  average  distance  between  the  two  eyes  ;  but  in 
practice  it  is  better  to  increase  it  in  the  case  of  portraits 
or  other  near  objects  to  about  twelve  inches,  and  in  that 
of  view's  to  even  several  feet.    Brewster's  original  rule 
for  taking  stereoscopic  photographs,  was  to  place  the 
cameras  one  foot  apart  for  every  twenty-five  feet  of  dis- 
tance.   The  beautiful  stereoscopic  pictures  of  the  moon 
photographed  by  Mr.  Warren  de  la  Rue  were  taken  at 
more  than  1,000  miles'  distance,  in  order  to  obtain  the 
necessary  relief.    The  principle  of  the  stereoscope  has 
received  many  useful  applications  in  the  way  of  book 
illustrations,  art  teaching,  and  anatomical  demonstra- 
tion, and  has  thus  gained  a  position  among  philoso- 
phical instruments  that  it  did  not  at  first  possess. 

A  combination  of  the  principles  of  the  phenakisti- 
scope  (fig.  4)  and  stereoscope,  has  resulted  in  the 
invention  of  an  instrument  called  the  stereotrope.  A 
number  of  binocular  photographs  of  some  object  in 
motion — a  steam-engine,  for  instance — are  taken  when 


THE  STEREOSCOPE. 


24.1 


the  moving  parts  are  in  different  positions,  and  mounted 
on  two  revolving  discs,  the  images  being  combined  by 
means  of  a  pair  of  semi-lenses,  as  in  the  ordinary  re- 
fracting stereoscope. 

"We  cannot  leave  this  subject  without  describing  the 
pseudoscope,  also  the  invention  of  Professor  Wheat- 
stone.  If  a  stereoscopic  pair  of  photographs  of  some 
solid  body — a  ball,  for  instance — are  mounted  the  re- 
verse way,  that  is  to  say,  if  the  picture  intended  to  be 
looked  at  by  the  right  eye  is  placed  on  the  left,  the 
relief  of  the  object  will  be  reversed,  and  the  ball  will 
appear  as  a  hollow  hemisphere.  If,  therefore,  we  can 
by  means  of  lenses  or  prisms  cause  the  image  of  any 
natural  obje  t,  as  seen  by  the  right  eye,  to  be  conveyed 
to  the  left,  and  vice  versa,  we  shall  see  the  relief  reversed. 
A  conical  cap  will  appear  in  relief  as  a  cone,  a  globe 
will  look  like  a  hollow  sphere,  and  the  human  face  will 
take  the  semblance  of  the  inside  of  a  mask.  The  same 
deception  may  be  effected  by  looking  at  a  seal  through 
a  short-focused  lens,  so  that  the  image  shall  seem  re- 
versed. In  this  case,  the  light  coming  apparently  from 
the  wrong  side,  and  shining  on  the  parts  in  relief,  gives 
them  the  appearance  of  being  hollow.  An  intaglio 
will,  of  course,  appear  in  relief  when  so  looked  at. 
Photographs  of  gems  and  bas-reliefs  will  also  present 
a  pseudoscopic  appearance,  if  looked  at  in  a  light 
coming  from  the  opposite  side  to  that  in  which  they 
were  taken.  The  same  appearance  may  be  seen  some- 
times in  wall  papers  having  patterns  painted  in  strong 
relief. 


242 


THE  WONDERS  OF  OPTICS. 


CHAPTER  VIII. 

THE  CAMERA  OBSCURA  AND  CAMERA  LUCIDA. 

The  construction  of  the  camera  obscura  is  found- 
ed on  the  fact  that   the  rays  of  light,  when  col- 
lected into  a  point  either  by  being  passed  through  a  small 
hole  or  a  converging  lens,  form  an  image  of  the  objects 
from  which  they  proceed  at  the  point  of  meeting. 
This  may  be  readily  tried  by  piercing  the  shutter  of  a 
room  with  a  small  hole,  and  holding  a  piece  of  paper 
within  a  short  distance  of  it.    It  will  be  noticed  that 
the  smaller  the  hole  the  more  distant  will  be  the 
image  formed.    The  first  person  who  observed  this  fact 
was  John  Baptist  Porta,  an  Italian  philosopher  who 
lived  in  the  latter  part  of  the  seventeenth  century.  He 
noticed  that  when  a  screen  was  placed  opposite  a  small 
hole  in  the  shutter  of  his  room,  the  objects  outside  were 
depicted  on  it  in  a  reversed  position  with  moderate  dis- 
tinctness ;    but  that  when  a  biconvex  lens  was  placed 
over  the  hole,   the  picture  was  rendered  much  more 
distinct.    This  was  the  first  attempt  at  the  formation  of 
the  camera  obscura,  an  instrument  that  has  since  be- 
stowed such  incalculable  benefits  on  humanity. 

The  shape  of  the  images  so  formed  is  independent  of 
the  shape  of  the  opening,  which,  as  long  as  it  is  suffi- 
ciently small,  may  be  square,  oval,  or  triangular.  This 
may  be  easily  seen  when  the  sun  shines  through  the 
intervals  between  the  leaves  of  a  shady  avenue  or  bower 


THE  CAMERA  OBSCURA  AND  CAMERA  LUCIDA.  245 


of  trees.  The  image  of  the  sun  as  a  circular  patch  of 
light  is  seen  scattered  over  the  surface  of  the  ground, 
although  the  accidental  intervals  formed  by  the  leaves 
above  were  of  a  thousand  different  shapes.  These 
images  at  the  time  of  an  eclipse  of  the  sun  are  very 
surprising,  taking,  as  they  do,  the  form  of  a  crescent, 
more  or  less  large  according  to  the  magnitude  of  the 
eclipse. 

This  property  possessed  by  the  rays  of  light,  of  de- 
picting on  a.  screen  the  forms  and  colours  of  the  objects 
from  which  they  proceed  when  passed  through  a  small 
aperture  or  a  lens,  is  taken  advantage  of  in  most  places 
famous  for  their  natural  scenery.  The  apparatus  em- 
ployed for  this  purpose  is  comparatively  simple,  consist- 
ing merely  of  a  dark  wooden  hut,  with  a  whitened  table 
in  the  centre,  and  a  mirror  and  lens  in  the  apex  of  the 
roof.  In  fig.  70  we  have  a  section  of  a  camera  obscura 
of  this  kind.  The  mirror  and  lens  at  the  top  of  the 
apparatus  are  made  to  revolve,  so  as  to  bring  every  part 
of  the  landscape  into  view  in  turn.  A  camera  obscura 
in  a  position  commanding  a.  view  of  moving  objects, 
such  as  ships  sailing  to  and  fro,  or  the  busy  streets  of 
a  populous  town,  is  an  unending  source  of  amusement, 
and  may  be  easily  and  cheaply  constructed. 

The  camera  obscura  has  been  much  utilized  for  taking 
hasty  but  exact  sketches  of  various  places.  For  this 
purpose  it  is  made  very  light,  and  mounted  on  three  legs 
carrying  at  their  junction  a  flat  table,  whereon  is  placed 
the  paper  to  receive  the  drawing.  The  tripod  is  cov- 
ered with  a  black  curtain,  which,  falling  over  the  artist, 
effectually  excludes  all  the  rays  of  light  except  those 
which  pass  through  the  lens  and  are  reflected  downwards 
by  the  mirror.  In  the  better  kind  of  apparatus  the 
mirror  is  replaced  by  a  prism,  which  throws  a  clearer 
image  than  a  mirror  upon  the  screen. 

It  is  on  these  properties  of  the  camera  obscura  that 


246 


THE  WONDERS  OF  OPTICS. 


the  art  of  photography  was  founded.  Everybody  who 
saw  the  beautiful  images  formed  by  this  instrument  was 
struck  with  the  idea  that  by  some  means  or  other  they 
could  be  fixed  on  paper.  After  numberless  attempts  the 
long-wished-for  goal  was  at  length  arrived  at ;  and  now 
optics,  aided  by  chemistry,  is  enabled  to  depict  for  us 
natural  objects  of  every  kind,  from  the  distorted  limb 
of  the  hospital  patient  to  the  beautiful  forms  of  the 
queens  and  empresses  of  the  world — from  the  tiniest 
animalcule  to  the  great  sun  itself,  who  is  compelled  by 
the  might  of  science  to  paint  his  own  portrait  for  us 
with  all  his  faults  and  imperfections. 

The  lenses  used  for  photographic  purposes  have  only 
reached  their  present  state  of  perfection  after  ceaseless 
labours  of  the  philosophers  and  opticians  of  all  coun- 
tries. At  first  only  a  single  lens  was  used,  but  it  was 
found  that  the  rays  which  exercised  a  chemical  action 
did  not  meet  in  the  same  point  as  the  rays  of  light,  for 
it  must  be  remembered  that  it  is  not  the  light  we  see 
that  acts  upon  the  substances  used  in  photography,  but 
another  influence,  known  as  actinism.  It  was  also 
found  that  a  single  lens  would  not  give  a  flat  picture 
when  the  whole  of  its  apenure  was  used,  the  edges  of 
the  image  being  always  blurred  and  indistinct.  This 
latter  defect  was  found  to  be  partially  obviated  by  de- 
creasing the  opening,  but  this  remedy  shut  off  the  light 
and  prolonged  the  process.  Gradually  these  two  de- 
fects were  removed,  and  now  every  photographer,  no 
matter  how  humble,  is  possessed  of  a  lens  capable  of 
taking  a  clear  picture,  every  detail  of  which  is  perfectly 
distinct  and  faithful. 

The  camera  lucida  bears  a  great  analogy  to  the  ca- 
mera obscura  in  the  purpose  for  which  it  is  used,  though 
not  in  the  principle  on  which  it  is  constructed.  It  is 
employed,  like  the  preceding  instrument,  for  obtaining 
faithful  copies  of  a  landscape,  a  building,  or  even  of  an- 


THE  CAMERA  OBSCURA  AND  CAMERA  LUCIDA.  247 

other  drawing.  It  was  invented  by  Dr.  Wollaston,  in 
1804,  and  consists  of  a  little  four-sided  prism,  of  which 
fig.  71  is  a  section. 


Fig.  71.— Section  of  Camera  Lucida. 


The  angle  at  A  is  a  right  angle  ;  the  angle  B  measures 
67J°,  the  angle  c  135°,  and  the  angle  D  is,  of  course, 
equal  to  B.  It  is  mounted  on  a  sliding  foot,  so  that  it 
may  be  raised  or  lowered  at  will,  or  turned  in  a  hori- 
zontal direction.  The  path  of  the  rays  in  this  case  is 
easy  to  follow,  the  object  to  be  copied  being  placed  at 
L,  and  the  eye  at  I.  On  looking  downwards  the  image 
of  the  object  to  be  drawn  is  seen  on  the  paper;  and  if 
the  eye  is  placed  so  that  the  edge  of  the  prism  will  just 
cut  the  pupil  in  two,  the  paper  and  pencil  will  be  seen 
at  the  same  time.  It  will  be  seen  from  the  diagram, 
that  the  rays  proceeding  from  L  strike  on  the  surface  A 
B  at  right  angles,  and,  being  then  reflected  from  C  B, 
pass  upwards  again  to  point  E.  The  direction  of  the 
rays  is  in  reality  a  little  more  complicated  than  this. 
In  the  case  of  distant  objects  it  is  impossible  to  see 
both  the  object  and  the  pencil  at  the  same  time;  a  lens 
is  sometimes  introduced  at  I  to  modify  this  defect.  The 
original  instrument  has  also  been  modified  by  the  intro- 


.248 


THE   WONDERS  OF  OPTICS. 


duction  of  a  triangular  prism,  in  conjunction  with  plates 
of  coloured  glass,  but  the  difficulty  of  rendering  the 
image  and  the  paper  of  the  same  strength  is  very  grea  t. 
The  instrument  is  also  hard  to  use,  from  the  additional 
difficulty  of  always  keeping  the  head  in  the  same  posi- 
tion, for  the  least  movement  from  left  or  right  is  suffi- 
cient to  throw  the  whole  drawing  out. 

A  simple  camera  lucida  may  be  made  out  of  a  small 
piece  of  looking-glass,  mounted  at  an  angle  of  45°,  or 
half-way  between  the  horizontal  and  the  perpendicular. 
If  this  be  turned  towards  the  drawing  or  view  to  be 
copied,  and  the  left  eye  applied  to  the  mirror,  the  image 
of  the  object  will  be  seen  on  the  paper  below,  and  the 
pencil  may  be  guided  with  the  right.  The  proper  use 
of  this  simple  little  instrument  depends  in  a  great  mea- 
sure upon  the  focus  of  each  eye  being  the  same.  The 
light  falling  on  the  paper,  too,  requires  very  careful 
adjusting,  otherwise  the  brighter  object  will  eclipse  the 
other.  It  is  a  good  plan,  too,  to  whiten  the  pencil  or 
pen  used,  so  that  it  may  not  so  easily  be  lost  when  draw- 
ing the  brighter  parts  of  the  object.  We  have  seen 
excellent  drawings  made  from  plants  by  means  of  a 
little  instrument  of  this  kind,  which  simply  consisted 
of  a  piece  of  looking-glass  inserted  in  a  cork  stuck  in  a 
glass  bottle. 


THE  SPECTROSCOPE. 


249 


CHAPTER  IX. 

THE  SPECTROSCOPE. 

We  now  come  to  speak  of  an  instrument  which  may 
fairly  rank,  after  the  telescope  and  microscope,  as  one 
of  the  most  wonderful  discoveries  of  modern  optical 
science.  By  its  means  we  have  not  only  discovered 
four  new  elementary  bodies,  which  are  found  in  certain 
minerals  in  inconceivably  small  quantities,  but  we  have 
also  determined  the  chemical  composition  of  some  of 
the  remotest  stars  and  nebulae. 

In  1701  Newton  discovered  that  if  an  ordinary  ray 
of  white  light  was  admitted  through  a  small  hole  into  a 
dark  chamber,  and  thence  passed  through  a  triangular 
prism,  it  became  decomposed  into  a  coloured  band, 
known  as  the  solar  spectrum.  As  we  have  already  ex- 
plained that  this  decomposition  is  caused  by  the  different 
coloured  rays  that  make  up  white  light  being  bent  un- 
equally by  the  action  of  the  prism,  we  trust  the  follow- 
ing explanations  will  be  readily  understood.  In  1802 
Dr.  Wollaston,  an  English  philosopher,  discovered  that 
by  using  a  narrow  slit,  instead  of  a  round  hole,  the  re- 
sulting spectrum  was  no  longer  continuous,  but  was 
divided  at  intervals  by  dark  lines  extending  across  it  in 
a  direction  parallel  to  the  edges  of  the  prism.  These 
line's  attracted  considerable  attention  at  the  time,  but  it 
was  not  until  1815,  that  Fraunhofer,  an  optician  of 
Munich,  investigated  them  with  accuracy.    He  mapped 


250 


THE  WONDERS  OF  OPTICS. 


and  counted  no  less  than  six  hundred  of  them,  identify- 
ing eight  of  the  most  conspicuous  by  the  first  eight 
letters  of  the  alphabet.    Their  positions  are  as  follow : — 


The  designations  of  these  lines  have  been  retained  to 
the  present  day,  and  they  have  been  named  after  the 
Munich  philosopher,  being  known  as  Fraunhofer's  lines. 
They  are  to  be  seen  in  all  parts  of  the  spectrum,  and 
increase  in  number  and  fineness  according  as  the  width 
of  the  slit  through  which  the  light  passes  is  diminished. 
It  may  be  asked,  how  it  happens  that  they  increase  in 
proportion  to  the  narrowness  of  the  aperture  admitting 
the  light  ?  A  little  consideration  will  soon  show  the 
reason  of  this. 

When  a  beam  of  light  is  passed  through  a  hole  of,  let 
us  say,  the  eighth  of  an  inch  in  diameter  and  decom- 
posed by  a  prism,  the  spectrum  so  produced  is  imperfect, 
inasmuch  as  an  infinite  number  of  spectra  are  thus  su- 
perposed, and  for  this  reason,  that  the  rays  of  light 
entering  on  the  right  side  of  the  aperture  will  give  a 
spectrum  falling  in  a  different  place  to  that  formed  by 
the  rays  entering  on  the  left.  In  order,  therefore,  to 
diminish  the  confusion  caused  by  the  superposition  of  a 
number  of  spectra,  the  aperture  ought  to  be  reduced  to 
a  narrow  slit.  When  the  thin  slice  of  light  passing 
through  the  slit  is  decomposed  by  the  prism,  we  find 
that  not  only  is  the  purity  of  the  colours  greatly  in- 
creased, but  the  lines  in  question  make  their  appearance 
more  or  less  in  all  parts  of  the  coloured  band. 

These  lines  are  very  unequally  distributed,  some  being 
crowded  together  in  masses,  while  others  are  extremely 
faint,  and  are  separated  by  large  intervals.  Their 


A.  Beginning  of  red. 

B.  Middle  of  red. 

0.  Beginning  of  orange. 

D.  Middle  of  yellow. 


E.  Middle  of  green. 

F.  Beginning  of  blue. 

G.  Middle  of  indigo. 

H.  Middle  of  violet. 


THE  SPECTROSCOPE. 


251 


position  is  well  marked  and  determined,  no  matter  from 
what  source  we  obtain  our  beam  of  sunlight.  Whether 
the  spectrum  be  produced  from  the  sun  itself,  or  from 
the  reflected  light  proceeding  from  the  moon  or  planets, 
they  are  still  found  in  the  same  place  ;  only  that  in  the 
latter  case  they  are  not  so  numerous,  on  account  of  the 
light  being  much  fainter.  For  many  years  the  cause  of 
these  lines  remained  a  complete  mystery,  and  it  was  not 
until  Bunsen  and  Kirchhoff  undertook  their  investigation 
that  a  satisfactory  explanation  of  their  origin  was  ar- 
rived at.  In  order  to  explain  this,  we  must  consider 
briefly  the  properties  of  the  spectra  of  flames,  and  other 
luminous  bodies. 

If,  instead  of  the  light  of  the  sun,  we  examine  pris- 
matically  the  light  given  off  by  an  incandescent  body, 
such  as  a  white-hot  piece  of  platinum,  we  shall  find  that 
the  lines  seen  in  the  solar  spectrum  are  absent,  and  that 
we  have  a  continuous  band  of  coloured  light  quite  unin- 
terrupted by  dark  spaces  or  bands.  The  same  absence 
of  lines  is  seen  in  the  spectra  of  the  electric  light  and 
the  flame  of  an  ordinary  candle,  the  light  in  each  of 
these  cases  being  produced  by  particles  of  carbon  in  a 
state  of  vivid  incandescence.  But  if  we  examine  the 
flame  of  incandescent  gases,  we  shall  find  a  spectrum  of 
an  entirely  new  kind.  Thus  if  we  examine  an  ordinaiy 
gaslight  through  a  slit  with  a  prism,  we  shall  obtain  a 
continuous  spectrum,  in  consequence  of  the  luminous 
portion  of  the  flame  consisting  of  solid  carbon  in  a  state 
of  incandescence  ;  but  if  we  turn  down  the  flame,  so  as 
to  lessen  the  amount  of  carbon  to  be  burned,  we  shall 
find  the  whole  of  that  body  is  converted  into  feebly  lumi- 
nous gas,  giving  off  a  faint  reddish  blue  light.  If  we 
now  again  examine  it  in  the  same  manner,  we  shall  fii.d 
that  the  spectrum  produced  consists  of  black  spaces, 
here  and  there  crossed  by  a  few  faint  coloured  lines  or 
bands.    The  reason  of  this  is  obvious  :  in  the  faint  flame 


252 


THE  WONDERS  OF  OPTICS. 


caused  by  the  carbon  and  hydrogen  in  a  state  of  lumi- 
nous vapour,  which  only  have  a  few  of  the  colours  of  the 
spectrum,  which,  when  passed  through  the  prism,  fall 
into  their  proper  places.  All  substances  with  which  we 
are  acquainted  are  capable  of  being  converted  into  lu- 
minous vapour  by  means  of  heat,  and  when  thus  burnt 
produce  Hamas  of  more  or  less  faint  luminosity,  gene- 
rally characteristically  coloured.  A  piece  of  soda 
inserted  in  the  wick  of  a  spirit  lamp  gives  a  yellow  tinge 
to  the  flame ;  a  morsel  of  saltpetre  (nitrate  of  potash) 
or  nitrate  of  strontia  will  give  a  purple  and  crimson 
tint  respectively.  These  hues  are  caused  by  the  metals 
sodium,  potassium,  and  strontium  contained  in  these 
salts  being  converted  into  luminous  vapour.  On  ana- 
lyzing these  coloured  flames  with  a  prism,  as  before,  we 
should  find  in  the  case  of  the  soda  a  single  broad  yellow 
line,  situated  just  in  the  middle  of  the  yellow  portion  of 
the  spectrum,  the  rest  of  the  space  where  the  spectrum 
should  be  being  perfectly  dark.  The  reason  of  this  is 
pretty  simple.  Sodium  burns  with  a  pure  yellow  flame, 
consequently  when  passed  through  a  prism  it  cannot 
split  into  any  other  colours,  but  takes  its  place  in  the 
position  belonging  to  yellow  of  that  particular  hue. 
Were  it  a  little  more  orange  or  green  in  tint,  it  would 
take  its  place  nearer  to  the  red  or  violet  end  of  the 
spectrum.  The  light  from  saltpetre,  which  contains 
potassium  may  next  be  examined.  It  will  be  found  to 
tinge  the  flame  with  the  spirit-lamp  of  a  beautiful  purple. 
We  can  almost  guess  what  will  happen  when  this  flame 
is  submitted  to  the  action  of  the  prism.  We  shall  find 
that  the  purple  light  emitted  will  split  into  red  and 
violet,  which  will  immediately  arrange  themselves  in 
their  proper  positions  according  to  their  hues.  If  in 
like  manner  we  substitute  nitrate  of  strontia  for  salt- 
petre, we  shall  get  a  splendid  crimson  flame  which  if? 
decomposed  by  the  prism  into  red,  orange,  or  blue. 


THE  SPECTROSCOPE. 


253 


On  submitting  the  compounds  of  the  other  elements 
to  the  same  tests,  we  shall  find  that  each  of  them,  when 
converted  into  luminous  gas,  is  capable  of  producing 
coloured  lines  of  various  kinds  when  the  light  of  their 
flames  is  passed  through  a  prism.  If,  therefore,  wTe  had 
a  number  of  salts  of  whose  composition  we  were  igno- 
rant, all  we  need  do  is  to  burn  them  in  a  spirit-lamp, 
and  by  the  number  and  position  in  the  lines  of  their 
spectra  we  should  be  able  to  tell  immediately  of  what 
they  were  composed. 

-  The  spectra  of  nearly  all  the  elements  capable  of 
being  connected  with  luminous  gas  have  been  determined 
with  great  accuracy.  Perhaps  the  number  and  position 
of  the  lines  of  a  few  spectra  will  be  interesting  to  the 
student. 

Sodium. — This  is  the  metallic  base  of  soda  salts,  and 
gives  a  double  bright  yellow  line  in  the  middle  of  the 
yellow. 

Potassium. — The  base  of  the  various  salts  of  potash. 
It  gives  one  line  in  the  extreme  red,  one  in  the  middle 
of  the  red,  one  in  the  violet,  and  a  peculiar  glow  in  the 
centre  of  the  spectrum. 

Strontium. — The  base  of  the  strontia  salts,  of  which 
the  nitrate  is  used  as  the  principal  ingredient  in  the  red 
fire  of  the  theatres.  It  gives  a  group  of  lines  in  the 
red  and  orange,  and  a  beautiful  blue  one  in  the  middle 
of  the  blue. 

Barium. — The  base  of  the  baryta  salts,  one  of  which 
is  used  in  making  green  fire.  It  gives  several  strong 
lines  in  the  green,  and  a  few  in  the  red,  orange,  and 
yellow. 

After  the  position  of  the  spectral  lines  of  most  of  the 
elements  had  been  discovered,  Messrs.  Bunsen  and 
KirchhofF  were  one  day  examining  the  saline  deposit  of 
a  spring  which  issues  from  the  earth  near  Durkheim,  in 


254 


THE  WONDERS  OF  OPTICS. 


the  Palatinate,  and  were  surprised  to  find  that  a  blue 
line  belonging  to  no  known  metal  made  its  appearance 
In  addition  to  the  potassium,  sodium,  and  other  lines 
produced  by  the  saline  ingredients  of  the  water.  These 
philosophers  immediately  concluded  that  the  unknown 
line  was  caused  by  an  unknown  metal,  and  they  at  once 
set  to  work  to  obtain  a  larger  quantity  of  the  saline 
residue  from  the  spring.  They  evaporated  down  no  less 
than  forty  tons  of  water,  and  succeeded  in  isolating  the 
new  substance,  which  turned  out  to  be  a  metal  resem- 
bling potassium.  While  examining  the  residue  more 
carefully,  a  new,  dark  red  line,  beyond  that  belonging 
to  potassium,  was  discovered,  pointing  to  the  existence 
of  a  second  new  element,  which  was  also  afterwards  ob- 
tained in  the  pure  state.  These  two  new  metals,  which 
closely  resemble  potassium  in  their  properties,  were 
named  in  accordance  with  the  lines  given  by  them  when 
converted  into  luminous  gas.  The  first  was  called  cae- 
sium, from  ccesius,  Lat.  light  blue;  and  the  other,  rubi- 
dium, from  rubidus,  Lat.  dark  red.  Since  the  publica- 
tion of  MM.  Bunsen  and  KirchhofPs  experiments,  these 
two  elements  have  been  found  in  comparatively  large 
quantities  in  various  minerals,  and  these  properties  have 
been  closely  studied. 

Spectrum  analysis  has  yielded  us  two  more  new 
metals  since  first  these  philosophers  applied  the  prism 
to  the  determination  of  the  chemical  composition  of 
various  bodies.  Mr.  W.  Crookes,  F.R.S.,  an  English 
chemist  of  eminence,  while  examining  the  flame  of  a 
deposit  obtained  during  the  manufacture  of  sulphuric 
acid  from  a  certain  sulphur  mineral  found  in  the  Hartz 
mountains,  perceived  a  brilliant  green  line  with  which 
he  was  previously  unacquainted,  which  quickly  flashed 
into  view,  and  then  disappeared.  After  numerous  ex- 
periments on  various  other  minerals  (for  the  deposit  he  had 
first  experimented  upon  only  yielded  him  a  few  grains 


THE  SPECTROSCOPE. 


255 


of  the  new  body),  Mr.  Crookes  succeeded  in  discovering 
a  comparatively  large  quantity  of  it  in  a  sulphur  mineral 
found  in  Belgium.  The  new  element  was  found  to  be 
a  heavy  metal,  closely  resembling  lead  in  its  properties. 
It  was  named  by  the  discoverer,  thallium,  from  the 
Greek  word  thallos,  a  green  twig,  from  the  brilliancy  of 
the  single  green  line  that  indicates  its  presence.  In 
like  manner,  Messrs.  Reich  and  Eichter  have  discovered 
a  fourth  new  metal,  which  has  been  named  indium,  from 
its  principal  lines  being  found  in  the  centre  of  the  indi- 
go of  the  spectrum. 

The  delicacy  of  spectrum  analysis  may  be  imagined 
from  the  fact  that  a  quantity  of  sodium  amounting  to 
less  than  the  two-millionth  of  a  grain  can  be  detected  by 
its  means.  Indeed,  it  has  taught  us  that  sodium  in  one 
form  or  other  exists  almost  everywhere.  This  mode 
of  analysis  is  only  serviceable  to  indicate  the  composi- 
tion of  any  salt  or  other  substance,  the  quantities  of  the 
different  elements  found  by  its  use  having  no  influence 
on  the  appearances  brought  out  by  the  prism.  Thus, 
asubstance  which  has  only  been  contaminated  with  sodium 
from  being  handled  by  warm  fingers,  will  show  the  yel- 
low bands  as  strongly  as  it  it  contained  a  large  propor- 
tion of  that  metal. 

For  ordinary  experiments  in  spectrum  analysis  the 
apparatus  used  is  very  simple.  It  consists  of  a  tube 
with  a  fine  slit  at  one  end,  and  a  convex  lens  at  the 
other,  for  concentrating  the  light  from  the  coloured 
flame  upon  the  centre  of  the  prism.  After  the  light 
passes  through  the  prism,  it  is  examined  by  a  small 
telescope  of  low  magnifying  power.  The  lamp  used 
may  be  either  a  spirit-lamp  or  a  colourless  gas  flame 
into  which  the  substance  to  be  examined  is  introduced 
upon  a  platinum  wire. 

We  now  come  to  another  very  important  discovery, 
made  by  means  of  our  prism  and  narrow  slit — the 


256 


THE  WONDERS  OF  OPTICS. 


determination  of  the  composition  of  the  photosphere  or 
mass  of  luminous  vapour  surrounding  the  body  of  the 
sun. 

A  simple  experiment  will  show  how  this  brilliant  dis- 
covery was  arrived  at.  The  light  of  a  candle  or  other 
flame  containing  incandescent  solid  matter  is  passed 
through  the  spectroscope,  and  is  found  to  decompose 
into  a  continuous  spectrum,  uninterrupted  by  dark  lines. 
Between  the  light  and  the  slit  a  spirit-lamp  is  placed, 
but  no  difference  in  the  appearance  of  the  spectrum  is 
perceived.  Introduce,  however,  the  smallest  portion  of 
a  soda  salt  into  the  non-luminous  flame  of  the  second- 
lamp,  and  a  broad  black  line  is  immediately  seen,  cross- 
ing the  middle  of  the  yellow  portion  of  the  band  of  co- 
lour. Remove  the  sodium  flame  and  the  band  disap- 
pears ;  but  do  the  same  with  the  lamp  producing  the 
spectrum,  and  the  spectrum  of  course  disappears,  and 
the  dark  band  caused  by  the  sodium  flame  is  changed 
to  the  yellow  line  produced  by  that  metal.  The  same 
experiments  may  be  tried  with  potassium,  strontium, 
and  other  metals  ;  and  we  shall  always  find  that  when 
a  coloured  flame  is  introduced  between  an  incandescent 
solid  and  its  continuous  spectrum,  it  produces  a  series 
of  black  lines  corresponding  to  the  substances  by  which 
it  is  coloured.  Thallium,  in  like  manner,  would  give  a 
black  band  in  the  middle  of  the  green,  and  indium  a 
similar  one  in  the  indigo.    (Fig.  6,  Frontispiece.) 

The  exaci;  position  of  the  black  band  in  the  middle  of 
the  yellow  is  shown  in  the  coloured  figurq  of  the  spec- 
trum so  beautifully  printed  in  the  frontispiece  of  this 
book,  and  it  has  been  found  to  correspond  exactly  with 
the  dark  line  D  of  the  solar  spectrum.  The  inference 
from  this  fact  is  obvious.  The  incandescent  portion  of 
the  sun  gives  off*  light  corresponding  in  its  properties  to 
that  emitted  by  the  solid  matter  contained  in  the  candle 
flame,  but  the  photosphere  containing  the  vapour  of  so- 


THE  SPECTROSCOPE. 


257 


dium  cuts  off  that  portion  corresponding  to  the  sodium 
line.  Accurate  measurements  prove  that  numberless 
other  lines  occurring  in  the  solar  spectrum  are  due  to 
the  vapours  of  other  well  known  metals  existing  on  the 
earth.  Amongst  these  may  be  mentioned  potassium, 
calcium  (the  base  of  lime),  iron,  nickel,  chromium,  and 
several  others.  This  discovery  with  regard  to  the  sun 
has  resulted  in  the  spectral  examination  of  a  large  num- 
ber of  the  fixed  stars  and  nebulae.  For  centuries  the 
fixed  stars  refused  to  answer  all  questions  put  to  them 
by  mortals.  The  telescope  showed  them  merely  as  bright 
points.  Their  nature  and  origin  remained  a  beautiful 
mystery,  until  Dr.  Miller,  Mr.  Huggins,  Father  Secchi, 
and  a  few  other  philosophers  interrogated  them  in  a 
manner  that  could  not  fail  to  draw  forth  an  answer. 
They  brought  their  light  within  range  of  their  prisms, 
and  forthwith  they  declared  themselves  to  be  suns  like 
our  own.  It  is  true  that  before  this  they  were  looked 
on  by  most  astronomers  as  bodies  analogous  to  our 
own  sun,  but  it  was  only  reasoning  from  analogy,  after 
all ;  but  we  are  now  able  to  assert  with  all  the  certainty 
that  is  compatible  with  human  fallibility  that  many  of 
these  heavenly  bodies  are  possessed  of  an  incandescent 
centre,  surrounded  by  a  photosphere  or  envelope  of 
gaseous  matter  in  a  luminous  condition.  It  would  be 
impossible  to  give  a  list  of  all  the  stars  that  have  been 
examined  up  to  the  present  time;  the  composition  of 
the  photospheres  of  a  few  must  therefore  suffice.  It  is 
singular  that  the  elements  hitherto  discovered  in  the 
stars  are  those  which  are  more  or  less  abundant  on  the 
earth.  Amongst  them  we  may  name  hydrogen,  nitro> 
gen,  sodium,  magnesium,  barium,  iron,  antimony,  bis- 
muth, tellurium,  and  mercury.  The  bright  star  in  the 
constellation  of  Orion  known  as  Betelgeux  is  one  of  the 
most  singular  in  composition,  the  lines  of  its  spectrum 
indicating  the  absence  of  hydrogen.    If,  as  Messrs. 

R 


258 


THE  WONDERS  OF  OPTICS. 


Huggins  and  Miller  suggest,  the  worlds  revolving  round 
this  star  are  also  deficient  in  this  element,  they  would 
be  without  water,  like  our  moon. 

Upon  a  very  clear  night  it  may  be  noticed  that  the 
stars  are  not  all  of  the  same  colour,  but  that  many  of 
them  appear  to  be  of  a  ruddy  or  yellowish  tint.  The 
cause  of  this  is  plainly  seen  when  they  are  submitted  to 
spectral  analysis.  Thus,  Sirius,  which  is  a  brilliant 
white  star,  shows  but  three  dark  lines,  while  one  of  the 
stars  in  the  constellation  of  Hercules  shows  several 
groups  of  bands  in  the  red,  blue,  and  green  portions  of 
its  spectrum,  fully  accounting  for  its  orange  tint. 

The  double  star  /3  Cygni  is  a  very  beautiful  example 
of  the  distribution  of  colour  between  two  members  of  a 
stellar  group.  One  star  shows  a  strong  spectrum  with 
the  blue  and  violet  portions  almost  totally  blotted  out, 
while  its  companion  is  similarly  circumstanced  with  re- 
spect to  the  yellow  and  orange  portions  of  its  spectrum. 
The  colour  of  one  is  consequently  orange,  while  the 
other  is  of  a  delicate  blue.  If  these  stars  are  the 
principal  members  of  a  system,  the  alternation  of  blue 
and  orange  days  must  be  indeed  a  singular  phenomenon 
to  those  who  inhabit  their  planets. 

In  some  of  the  stars  lines  have  been  discovered 
which  do  not  possess  any  equivalent  amongst  those  pro- 
duced by  terrestrial  matter  ;  they  consequently  contain 
elements  of  which  we  know  nothing ;  at  the  same  time, 
however,  it  has  been  found  that  terrestrial  elements 
exist  in  some  of  the  remote  nebulse,  which  are  so  dis- 
tant that  their  light  takes  many  thousands  of  years  to 
reach  our  earth. 

Spectrum  analysis  has  decided  the  grand  question  of 
the  physical  composition  of  the  nebulae.  Those  bodies 
were  supposed,  with  some  reason,  to  be  aggregations  of 
stars,  like  our  Milky  Way,  which  only  required  tele- 
scopes of  sufficient  power  to  resolve  them.    That  they 


THE  SPECTROSCOPE. 


259 


partly  consist  of  gaseous  matter  in  a  luminous  condition 
is  evidenced  by  their  showing  a  series  of  bright  lines  in 
the  spectroscope,  exactly  like  those  produced  by  terres- 
trial gases.  Their  light  is  therefore  not  emitted  by  a 
solid  or  liquid  incandescent  body,  but  by  a  glowing  gas. 
The  lines  mentioned  by  Messrs.  Huggins  and  Miller 
showed  that  the  nebula  in  the  sword-handle  of  Orion 
consists  of  hydrogen  and  nitrogen  in  a  state  of  luminous 
incandescence.  Not  the  slightest  trace  of  a  continuous 
spectrum  can  be  detected  in  the  light  emanating  from 
this  body ;  consequently,  according  to  present  hypo- 
theses, it  contains  no  solid  matter  at  all.  A  number  of 
other  nebulae  have  given  similar  results. 

There  are  numerous  star  clusters  which,  unlike  the 
true  nebulae,  give  continuous  spectra  when  their  light 
is  submitted  to  the  action  of  the  prism.  Of  these  may 
be  specially  mentioned  the  great  clusters  in  Andromeda 
and  Hercules,  which  give  continuous  spectra,  inter- 
rupted by  dark  bands  on  the  red  and  orange.  The 
light  thrown  by  these  experiments  upon  the  nebular 
hypotheses  of  Sir  William  Herschel,  who  considered 
that  true  nebulae  consisted  of  the  primordial  gaseous 
matter  out  of  which  suns  and  stars  have  been  elabo- 
rated, is  very  great,  and  will  be  appreciated  even  by 
those  whose  knowledge  of  astronomy  is  small. 

Spectral  analysis  has  also  been  the  means  of  our  wit- 
nessing a  celestial  conflagration,  and  understanding  the 
cause  of  this  marvellous  event.  It  is  well  known  to 
most  people  that  from  time  to  time  stars  have  suddenly 
burst  upon  us,  and  have  almost  as  suddenly  disappeared. 
The  theories  advanced  to  account  for  these  singular 
celestial  visitors,  have  been  more  numerous  than  satis- 
factory. In  May  1866,  a  star  of  the  second  magni- 
tude suddenly  burst  forth  in  the  Northern  Crown,  and 
was  almost  immediately  noticed  by  Mr.  Huggins  who 
brought  every  power  of  prism  and  telescope  to  bear 


260 


THE  WONDERS  OF  OPTICS. 


upon  this  extraordinary  celestial  phenomenon.  He 
found  the  spectrum  of  the  star  to  consist  of  two  distinct 
spectra,  one  being  formed  by  four  bright  lines,  the 
ttther  analogous  to  the  spectra  of  the  sun  and  stars. 
Consequently  two  kinds  of  light  were  given  off  by  this 
star ;  one  forming  a  series  of  bright  lines  indicative  of 
luminous  gas,  the  other  consisting  of  a  continuous 
spectrum,  crossed  by  dark  lines,  showing  the  existence 
of  a  solid  body  in  a  state  of  incandescence,  surrounded 
by  a  photosphere  of  luminous  vapours.  Two  of  the 
bright  lines  undoubtedly  showed  the  presence  of 
hydrogen  in  a  state  of  illumination,  the  great  bright- 
ness of  the  lines  indicating  that  the  burning  gas  was 
hotter  than  the  photosphere.  These  facts  taken  in 
conjunction  with  the  suddenness  of  the  outburst  in  the 
star,  and  its  immediate  decline  in  brightness  from  the 
second  down  to  the  eighth  magnitude  in  twelve  days, 
suggest  the  startling  speculation  that  the  star  had  be- 
come suddenly  wrapped  in  the  flames  of  burning 
hydrogen,  consequent  possibly  on  some  violent  con- 
vulsion in  the  interior  of  the  star  having  set  free 
enormous  quantities  of  this  gas.  As  the  free  hydrogen 
became  exhausted,  the  spectrum  showing  the  bright 
lines  gradually  waned  until  the  star  decreased  in  bril- 
liancy. It  must  not  be  forgotten  that  the  event  seen  by 
Mr.  Huggins  occurred  many  years  ago,  and  that  the 
light  emitted  by  this  marvellous  celestial  convulsion  has 
been  travelling  to  us  ever  since. 

Comets  and  meteors  have  been  submitted  to  the  test 
of  spectral  analysis.  The  former  erratic  visitors  have 
been  but  few  and  small  since  stellar  spectrum  analysis 
has  been  perfected.  In  January  1866,  Mr.  Huggins 
brought  his  apparatus  to  bear  upon  a  small  comet, 
which  gave  a  somewhat  unexpected  result.  When  the 
object  was  viewed  in  the  spectroscope,  two  spectra  were 
distinguishable — a  very  faint  continuous  spectrum  of 


THE  SPECTROSCOPE. 


261 


the  tail,  showing  that  it  reflected  solar  light,  and  a 
bright  space  towards  the  centre  of  the  spectrum,  indi- 
cating that  the  nucleous  was  self-luminous  and  gaseous. 

Mr.  Alexander  Herschel — the  nephew  and  the  grand- 
son of  Sir  John  and  Sir  William  Herschel — has  recently 
succeeded  in  obtaining  indications  of  the  composition 
of  the  meteors  that  people  the  heavens  in  the  months 
of  August  and  November.  The  principal  result  of  his 
observations  appears  to  be,  that  sodium  in  a  state  of 
luminous  vapour  is  present  in  the  trains  left  behind 
these  singular  bodies. 

Lightning  has  also  been  similarly  examined,  and  lines 
showing  that  hydrogen  and  nitrogen  were  rendered  lu- 
minous during  the  electrical  discharge,  were  seen  with 
great  distinctness.  In  fact,  the  applications  of  the 
prism  to  scientific  discovery  are  almost  endless,  and 
in  describing  them  it  is  difficult  to  tell  where  to  draw 
the  line. 

Before  quitting  this  subject,  it  will  be  as  well  to  say 
a  few  words  on  the  fluorescent  rays  of  the  spectrum,  to 
which  allusion  has  already  been  made  towards  the  end 
of  Chapter  IV.,  Part  II.  It  was  there  said  that  the 
chemical  power  of  the  spectrum  extends  to  some  dis- 
tance beyond  the  extreme  violet,  a  fact  that  may  be 
readily  proved  by  exposing  a  piece  of  photographic 
paper  to  the  action  of  the  dark  portion  of  the  spectrum. 
Professor  Stokes  found  that  there  were  means  of  render- 
ing these  rays  visible  to  the  eye  by  altering  their  rate 
of  vibration.  This  he  found  was  possible  by  passing 
them  through  the  solutions  of  certain  substances,  such 
as  sulphate  of  quinine,  horse-chestnut  bark,  &c.  We 
have  already  said,  that  light  vibrating  at  the  rate  of 
from  458  to  727  billion  times  a  second,  was  capable  of 
exciting  luminous  sensations  upon  the  optic  nerve.  The 
latter  is  the  rate  of  vibration  of  the  extreme  violet  ray, 
and  it  has  been  found  that  the  eyes  of  many  persons  are 


262 


THE  WONDERS  OF  OPTICS. 


not  sufficiently  sensitive  to  be  influenced  by  it ;  it  is, 
therefore,  just  probable  that  there  are  animals  whose 
eyes  are  so  much  more  sensitive  than  ours,  that  they 
can  see  rays  that  exist  far  beyond  those  seen  by  us. 
Now,  as  difference  of  colour  is  produced  by  difference 
in  the  rate  of  vibration,  it  follows  that  those  whose  eyes 
are  sensitive  enough  to  perceive  the  extreme  violet  rays, 
see  tints  of  violet  that  are  inappreciable  by  others. 

The  power  of  sulphate  of  quinine  in  reducing  the 
luminous  vibrations  is  easily  seen  by  passing  a  tube 
filled  with  the  solution  successively  through  each  of  the 
colours  of  the  spectrum  formed  by  a  quartz  prism ;  the 
ordinary  colours  will  pass  through  the  liquid  as  if  it 
were  simply  water,  but  on  arriving  near  the  violet 
extremity  a  gleam  of  pale  blue  light  will  shoot  across 
the  tube,  and  continue  to  increase.  As  it  is  moved 
onwards  the  light  will  gradually  die  away,  until  a  point 
is  reached  nearly  equal  in  length  to  the  whole  of  the 
visible  spectrum,  when  it  will  disappear  altogether.  It 
is  somewhat  singular  that  no  substance  has  yet  been 
found  that  will  increase  the  refrangibility  of  the  dark 
rays  beyond  the  red  end  of  the  spectrum.  There  are 
many  artificial  flames  which  produce  this  dark  light  (if 
we  may  use  such  a  paradoxical  expression)  in  greater 
quantity  than  the  sun,  whose  light  is  no  doubt  greatly 
deteriorated  in  this  respect  during  its  passage  through  the 
atmosphere.  The  substance  of  which  the  prism  is  made 
also  greatly  influences  the  length  of  the  invisible  por- 
tion of  the  spectrum.  By  using  a  quartz  prism  and 
lenses  of  the  same  material  Professor  Stokes,  found  that 
the  spectrum  of  the  electric  light  could  be  traced  for  a 
distance  equal  to  six  times  that  of  the  visible  portion. 

The  action  of  certain  substances  in  rendering  the  in- 
visible rays  of  light  perceptible  may  be  easily  shown 
by  any  one  possessing  a  horse-chestnut  tree.  A  weak 
decoction  of  the  inner  portion  of  the  bark  having  been 


THE  SPECTROSCOPE. 


263 


made  and  filtered  through  blotting-paper,  or  at  any  rate 
allowed  to  settle,  the  room  is  made  quite  dark  and  a 
piece  of  common  brimstone  is  ignited.  The  pale  blue 
light  given  off  is  comparatively  feeble,  but  it  is  very 
rich  in  the  ultra-violet  rays ;  consequently,  when  the 
infusion  of  horse-chestnut  bark  is  poured  into  a  tall  jar 
of  water,  beautiful  waves  of  phosphorescent  light  are 
seen  flashing  backwards  and  forwards  as  the  two  liquids 
mingle.  The  tincture  of  stramonium  is  also  possessed 
of  this  property,  and  characters  traced  on  paper  with 
it,  although  nearly  invisible  by  ordinary  daylight,  appear 
distinctly  when  examined  by  the  light  of  burning 
sulphur. 


264 


THE  WONDERS  OE  OPTICS. 


CHAPTER  X. 

SPECTRES — THE   GHOST  ILLUSION* 

We  close  our  account  of  the  wonders  of  optics  by  a 
description  of  the  ghost  illusion,  which  has  been  exhi- 
bited with  such  great  success  by  M.  Robin,  the  well- 
known  French  conjurer,  Mr.  Pepper,  the  enterprising 
manager  of  the  Royal  Polytechnic  Institution,  and  seve- 
ral others.  Before  doing  so,  however,  we  will  say  a  few 
words  on  those  unpleasant  visitations  known  as  spectres, 
to  which  some  people  are  liable,  either  through  an  over- 
worked brain  or  some  organic  disease. 

The  peculiar  appearances  known  as  spectres  in  optics 
are  certain  illusions  of  vision  in  which  an  object  is  ap- 
parently presented  to  the  view  whi3h  does  not  really 
exist.  In  such  cases  either  the  brain,  the  retina,  or  the 
optic  nerve  are  unnaturally  excited,  and  made  sensitive 
to  an  appearance  that,  physically  speaking,  does  not 
exist.  There  is  such  a  close  connexion  between  the 
senses  and  the  mind,  that  we  continually,  and  without 
knowing  it,  transfer  to  the  physical  world  that  which 
belongs  to  the  domain  of  thought.  A  picture  which  has 
struck  us  during  the  day  will  reappear  to  us  at  night 
during  sleep,  with  every  detail  perfect,  or  possibly  under 
a  form  modified  by  the  capricious  wanderings  of  our 
thoughts.  A  sudden  fright  may  sometimes  be  the  cause 
of  optical  illusions  which  will  pursue  us  unceasingly. 
Fear,  despair,  passion,  ambition,  and  other  violent  men- 


SPECTRES — THE  GHOST  ILLUSION. 


265 


tal  phases,  are  capable  of  evoking  images  closely  con- 
nected with  the  state  of  our  brain,  appearances  that  we 
often  take  for  realities,  and  whose  truths  we  have  to 
test  by  our  faculty  of  reasoning,  before  we  can  set  them 
down  as  positive  illusions.  "  In  the  most  insignificant 
phenomena,"  says  Sir  David  Brewster,  "we  find  that 
the  retina  is  so  powerfully  influenced  by  exterior  im- 
pressions as  to  retain  the  images  of  visible  objects  for  a 
long  time  after  they  have  passed  out  of  sight ;  besides, 
this  portion  of  the  eye  is  so  strongly  influenced  by  local 
impressions  of  which  we  know  neither  the  nature  nor 
the  origin,  that  we  see  the  shapeless  forms  of  coloured 
light  moving  about  in  the  dark.  In  fact  we  have,  in 
the  cases  of  Newton  and  many  others,  examples  of  the 
ease  with  which  the  imagination  revivifies  the  images 
of  luminous  objects  for  months  or  even  years,  after 
these  impressions  took  place.  After  the  occurrence  of 
such  phenomena,  the  mind  can  readily  comprehend  how 
thin  is  the  division  that  separates  reality  from  those 
spectral  illusions  which  during  a  particular  state  of 
health  have  afflicted  the  most  intelligent  men,  not  merely 
those  belonging  to  the  community  at  large,  but  also 
the  most  learned  philosophers." 

Spectres  may  properly  be  divided  into  two  classes, 
those  which  may  be  termed  subjective,  which  result  from 
some  unnatural  action  of  our  minds  or  bodies,  and 
which  properly  belong  to  the  science  of  physiology,  and 
those  which  may  be  called  objective,  which  are  caused 
by  some  peculiar  illusion  acting  on  us  from  without. 
We  shall  pass  lightly  over  the  first,  illustrating  them 
by  a  single  example,  while  we  shall  pay  more  serious 
attention  to  those  belonging  to  the  second  class. 

Sir  Walter  Scott,  in  his  Letters  on  Demonology  and 
Witchcraft,  mentions  a  remarkable  instance  of  the  first 
order  of  spectres.  A  doctor  of  eminence  was  called  in 
to  attend  a  gentleman  who  occupied  a  high  place  in  a 


266 


THE  WONDERS  OF  OPTICS. 


particular  department  connected  with  the  administration 
of  justice.    Until  the  time  that  the  physician's  services 
became  necessary,  he  had  shown  strong  common  sense 
and  extraordinary  firmness  and  integrity  in  every  case 
in  which  he  had  been  called  upon  to  arbitrate.  But  after 
a  certain  epoch  his  temper  became  saddened,  although 
his  mind  preserved  its  habitual  strength  and  calmness. 
At  the  same  time,  the  feebleness  of  his  pulse,  the  loss 
of  appetite,  and  impaired  digestion  seemed  to  point  out 
to  his  medical  adviser  the  existence  of  some  serious 
source  of  disturbance.     At  first  the  sick  man  seemed 
inclined  to  keep  the  cause  of  the  change  in  his  health  a 
profound  secret;  but  his  melancholy  bearing,  confused 
answers,  and  the  badly  disguised  constraint  with  which 
he  sharply  replied  to  the  interrogations  of  the  doctor, 
caused  the  latter  to  seek  for  information  as  to  the  cause 
of  the  disorder  in  other  directions.    He  made  minute 
inquiries  of  the  various  members  of  his  unhappy  pa- 
tient's family,  but  he  could  obtain  no  explanation  of  the 
mystery.     Every  one  was  lost  in  conjecture  as  to  the 
reason  of  the  alarming  condition  of  the  patient,  which 
did  not  appear  to  be  justified  by  any  loss  of  fortune  or 
beloved  friends.    His  age  rendered  the  idea  of  an  unsuc- 
cessful love  affair  improbable,  and  his  known  integrity 
precluded  the  possibility  of  remorse.   The  doctor  accord- 
ingly was  compelled  to  return  once  more  to  the  straight 
road,  and  he  used  the  most  serious  arguments  with  his  pa- 
tient to  induce  him  to  conquer  his  obstinacy.     At  last 
the  doctor's  efforts  took  effect ;  the  patient  allowed  him- 
self to  be  convinced,  and  manifested  his  desire  to  open 
his  mind  frankly  to  the  doctor.   They  were  accordingly 
left  alone,  all  the  doors  were  securely  fastened,  and  the 
patient  made  the  following  singular  avowal. 

"  You  cannot  be  more  firmly  convinced,  my  dear 
friend,  than  I  am  myself,  that  I  am  on  the  eve  of  death, 
crushed  by  the  fatal  malady  which  has  dried  up  the 


SPECTRES — THE  GHOST  ILLUSION.  267 


sources  of  my  life.  You  remember,  without  doubt,  the 
disease  of  which  the  Duke  of  Olivarez  died  in  Spain  ?" 

"From  the  idea,"  replied  the  doctor,  "  that  he  was 
pursued  by  an  apparition  in  whose  existence  he  did  not 
believe^  and  he  died  from  the  continual  presence  of  this 
imaginary  vision  weighing  down  his  strength,  and  break- 
ing his  heart." 

"  Well,  my  dear  doctor,"  the  patient  went  on,  "  I  am 
in  the  same  condition,  and  the  presence  of  the  vision 
that  persecutes  me  is  so  painful  and  frightful,  that  my 
reason  is  totally  helpless  in  controlling  the  effects  of  my 
imagination,  and  I  feel  that  I  am  dying  from  the  effects 
of  an  imaginary  illness.  My  visions  began  two  or  three 
years  since.  At  first  I  found  myself  embarrassed  from 
time  to  time  by  the  presence  of  a  great  cat,  which  ap- 
peared and  disappeared  I  knew  not  how.  But  at  last 
the  truth  flashed  across  my  mind,  and  I  was  compelled 
to  look  upon  the  creature,  not  as  an  ordinary  domestic 
animal,  but  as  a  vision  which  had  its  origin  in  some  de- 
rangement of  the  organs  of  sight  or  in  my  imagination. 
I  have  no  antipathy  to  cats,  in  fact  I  am  rather  fond  of 
them,  so  I  endured  the  presence  of  my  imaginary  com- 
panion so  well  that  at  last  I  treated  the  whole  affair 
with  indifference.  But  at  the  end  of  several  months 
the  cat  disappeared,  and  was  replaced  by  a  spectre  of 
greater  importance,  and  whose  exterior  was,  to  say  the 
least  of  it,  very  imposing.  It  was  neither  more  nor  less 
than  one  of  the  high  officials  of  the  House  of  Lords,  in 
the  full  dress  belonging  to  his  dignity. 

"  This*  personage,  who  was  in  court  dress,  with  a  bag- 
wig  on  his  head,  and  a  sword  by  his  side,  his  coat  splen- 
didly embroidered  and  his  ehajieau  bras  under  his  arm, 
glided  along  by  my  side  like  a  shadow.  Whether  I  was 
in  my  own  house  or  elsewhere,  he  mounted  the  stairs 
before  me,  as  if  to  announce  my  coming.  Sometimes  he 
seemed  to  mix  with  the  company,  although  it  was  evident 


268 


THE  WONDERS  OF  OPTICS. 


that  no  one  remarked  his  presence,  and  I  was  the  sole 
witness  of  the  chimerical  honours  that  this  imaginary 
individual  seemed  to  render  to  me.  This  phantasy  of 
my  brain  did  not  make  a  very  strong  impression  on  me, 
although  it  made  me  conceive  doubts  as  to  the  state  of 
my  health,  and  the  effects  it  would  produce  upon  my 
reason. 

"  This  second  phase  of  my  malady,  like  the  first,  also 
came  to  an  end.  Some  months  after,  the  usher  of  the 
Upper  House  ceased  showing  himself,  and  he  was  re- 
placed by  an  apparition  that  was  at  once  wearing  to  the 
mind  and  terrible  to  the  sight.  It  was  a  skeleton. 
Whether  I  was  alone  or  in  company  this  frightful  image 
of  death  never  quitted  me ;  it  dogged  my  footsteps  and 
followed  me  everywhere,  and  seemed  to  be  a  shadow 
inseparable  from  myself.  It  was  in  vain  that  I  repeated 
to  myself  a  hundred  times  over  that  the  vision  was  not 
real,  and  was  only  an  illusion  of  my  senses.  The  rea- 
soning of  philosophy  and  my  religious  principles,  strong 
though  they  are,  are  powerless  to  triumph  over  the  in- 
fluence that  besets  me,  and  I  feel  that  I  shall  die  a 
victim  to  this  cruel  evil.,, 

"It  seems  then/'  interrupted  the  doctor,  "  that  this 
skeleton  is  always  before  your  eyes  ?" 

"  It  is  my  evil  fate  to  see  it  continually  before  me." 

"In  which  case  it  is  at  this  moment  visible  to  your 
eyes  ?" 

"  It  is  at  present." 

"  And  in  what  part  of  the  room  do  you  imagine  that 
you  see  it  now  ?"  asked  the  doctor. 

"  At  the  foot  of  my  bed,"  replied  the  patient :  "  when 
the  curtains  are  half  open  I  can  see  it  place  itself  in  the 
empty  space  between  them." 

"  You  say  that  you  are  convinced  that  it  is  only  an  illu- 
sion," replied  the  doctor  ;  "have  you  the  firmness  to  con- 
vince yourself  of  it  positively  ?    Have  you  the  necessary 


Fig.  72. — The  Spectre.    An  optical  illusion. 


SPECTRES — GHOST  ILLUSION. 


269 


courage  to  get  up  and  go  and  place  yourself  in  the 
position  which  appears  to  be  occupied  by  the  spectre,  in 
order  to  demonstrate  to  yourself  positively  that  it  is 
only  a  vision  ?" 

The  unfortunate  man  sighed  and  shook  his  head. 

"Well,"  went  on  the  doctor,  "  let  us  try  another 
plan." 

He  quitted  the  chair  on  which  he  was  sitting,  at  the 
head  of  his  patients  bed,  and  placing  himself  between 
the  half  opened  curtains,  in  the  place  where  the  patient 
had  pointed  out  the  skeleton,  he  asked  if  the  apparition 
was  still  visible. 

"Not  the  whole  of  it,"  answered  the  patient,  "be- 
cause you  are  standing  between  him  and  me;  but  I  see 
his  skull  looking  at  me  over  your  shoulder." 

In  spite  of  his  philosophy,  the  learned  physician  could 
not  help  starting  to  hear  that  the  spectre  was  immedi- 
ately behind  him.  He  had  recourse  to  other  questions, 
and  tried  endless  remedies,  but  without  success.  The 
prostration  of  the  patient,  however,  increased,  and  he 
died  in  the  same  distress  of  mind  in  which  he  had  passed 
the  last  months  of  his  life.  This  example  is  a  sad  proof 
of  the  power  of  the  imagination  over  the  life  of  the  body 
even  when  the  terrors  endured  are  powerless  in  destroy- 
ing the  judgment  of  the  unfortunate  sufferer.  We  will 
say  more ;  men  who  have  the  strongest  nerves  are  not 
free  from  similar  illusions. 

The  second  kind  of  spectres,  in  which  the  science  of 
optics  plays  so  important  a  part,  is  the  result  of  the 
imagination  being  deceived  by  art  with  the  assistance  of 
science. 

These  spectres  are  displayed  in  the  ghost  trick  which 
has  been  practised  at  various  Parisian  theatres  for  a 
number  of  years,  with  very  great  success,  more  especially 
at  the  Theatres  du  Chdtelet  and  Dejazet.  The  Adelphi, 
in  London,  also  employed  Mr.  Pepper  to  heighten  the 


270 


THE  WONDERS  OF  OPTICS. 


effect  of  the  excellent  actingof  Mr.  Toole  and  Mrs.  Alfred 
Mellon,  in  the  dramatic  version  of  Dickens'  "  Haunted 
Man,"  by  the  introduction  of  various  spectral  effects. 
And  the  same  trick  was  also  called  into  requisition  with 
some  success  in  several  of  the  minor  theatres  in  New 
York  and  other  cities  of  the  United  States.  At  the 
Polytechnic,  in  London,  very  remarkable  effects  were 
produced,  and  few  who  ever  saw  them  will  forget  the 
surprise  they  felt  at  seeing  the  first  representation 
of  an  imponderable  ghost  endowed  with  motion,  and 
even  speech.  Amongst  the  most  successful  pro- 
ductions in  this  way  was  the  entertainment  of  M. 
Robin,  one  of  the  cleverest  of  the  many  successors  of 
the  great  Robert  Houdin,  the  prince  of  prestidigitators. 
M.  Robin  claims  to  be  the  inventor  of  the  ghost  illusion, 
and  to  have  shown  it  frequently  since  1847.  Whether 
this  be  so  or  not  it  is  not  our  business  to  decide,  but  we 
can  testify  that  his  exhibition  in  the  Boulevard  du 
Temple  drew  all  Paris  to  see  it.  Evening  after  evening 
he  not  only  "  called  spirits  from  the  vasty  deep,"  but 
"  made  them  come."  He  pierced  them  with  swords,  he 
fired  pistols  through  them,  and  he  made  them  appear 
and  disappear  at  his  slightest  wish.  He  showed  the 
Zouave  at  Inkermann,  lying  dead  amongst  a  heap  of 
slain,  who  at  the  familiar  sound  of  the  drum,  rose,  pale 
and  grave,  and  showed  the  bleeding  wounds  from  which 
he  died.  Amongst  other  scenes  shown  by  M.  Robin  was 
one  of  a  spectre  appearing  to  an  armed  man,  who  after 
trying  in  vain  to  shut  out  the  vision  from  his  sight  fires 
a  pistol  at  the  intruder.  Fig.  72  shows  the  scene  as  seen 
by  the  audience,  and  fig.  73,  the  method  by  which  the 
illusion  is  worked.  The  theatre  is  shown  in  section.  On 
the  left,  at  the  end,  are  seen  the  spectators;  on  the  right  is 
the  stage  upon  which  the  scene  is  represented.  Beneath 
the  stage  is  an  actor  clothed  in  white  to  personate  a 
ghost,  whose  image  is  reflected  by  the  glass  above. 


SPECTRES — GHOST  ILLUSION. 


275 


This  glass  is  placed  at  an  angle,  and  fills  up  the  whole 
of  the  front  of  the  stage,  the  edges  being  carefully  con- 
cealed by  curtains.  The  glass  of  course  must  be  of  a 
very  large  size,  and  should  be  of  the  very  best  quality, 
so  that  it  cannot  be  seen  by  the^  audience.  The  actor 
must  take  care  to  place  himself  in  such  a  position  as  to 
counteract  the  effect  produced  by  the  glass  being  placed 
at  an  angle.  At  first  the  cavalier  is  seen  sitting  at  a 
table.  After  soliloquizing  for  a  time  in  a  very  remorse- 
ful manner  touching  several  murders  that  he  has  com- 
mitted, the  ghost  of  one  of  his  victims  gradually  appears. 
This  is  effected  by  gently  turning  the  electric  light  upon 
the  concealed  actor.  The  aurderer  and  victim  parley  for 
a  short  time,  when  the  former,  being  unable  to  withstand 
the  reproaches  of  the  ghost  any  longer,  fires  a  pistol  at 
him  point-blank.  The  ball  of  course  takes  no  effect,  so 
the  villain  draws  a  sword,  but  before  it  has  left  its 
scabbard  the  spirit  of  the  victim  has  vanished  with  a 
mocking  laugh,  or,  in  other  words,  the  electric  light  is 
suddenly  turned  off.  Tb^  iaa/iiigement  of  the  light  is 
exceedingly  difficult  unaer  tnese  circumstances ;  the 
theatre,  the  stage,  and  the  portion  beneath  ought  to  be 
lighted  in  a  very  careful  manner,  for  if  either  is  too 
bright  or  too  dark  it  mars  the  whole  effect.  It  must  be 
remembered,  too,  that  the  person  performing  the  part  of 
the  spectre  and  the  real  actor  above  cannot  see  each 
other,  consequently  all  their  action  has  to  be  carried 
on  by  guess-work.  The  actor  below  has  to  walk  along 
an  inclined  plane,  keeping  himself  exactly  at  right  angles 
to  it.  Again,  the  movements  of  the  latter  are  obliged  to 
be  reversed;  for  the  cavalier  already  mentioned  drew 
his  sword  with  his  left  hand  in  order  that  the  reflected 
figure  should  appear  to  use  the  right. 

When  well  arranged,  the  ghost  trick  leaves  far  behind 
all  the  efforts  of  a  similar  nature  that  were  obtained  by 
the  ancients  in  the  way  of  magical  illusions.    It  is  also 


276 


THE  WONDERS  OF  OPTICS. 


incontestably  true,  contrary  to  what  some  people  have 
supposed,  that  they  were  unable  to  perform  this  illusion 
in  the  way  we  have  described,  for  they  were  ignorant  of 
the  method  of  manufacturing  and  polishing  glass  plates 
of  sufficient  size  and  clearness  for  the  purpose. 

The  production  of  living  but  impalpable  spectres  is 
thus  a  completely  modern  achievement,  as  we  have 
already  proved,  and  which  has  taken  its  place  amongst 
the  applications  of  science  to  stage  art,  to  the  total  ex- 
clusion of  all  effects  depending  for  their  production  on 
the  old-fashioned  phantasmagoria  and  magic  lantern. 


THE  END. 


V 


